samples: Bluetooth: hci_rpmsg: Fix stack sizes based on profiling

Fix stack size requirement in hci_rpmsg sample based on
profiling using thread analyzer.

Below is the observed stack usages now under Connected ISO
audio application on nrf5340_audio_dk_nrf5340_cpunet:
Thread analyze:
 0x21001968          : STACK: unused 248 usage 264 / 512 (51 %); CPU: 1 %
      : Total CPU cycles used: 102725
 HCI rpmsg TX        : STACK: unused 560 usage 592 / 1152 (51 %); CPU: 0 %
      : Total CPU cycles used: 73484
 BT RX               : STACK: unused 208 usage 432 / 640 (67 %); CPU: 2 %
      : Total CPU cycles used: 198386
 BT RX pri           : STACK: unused 200 usage 184 / 384 (47 %); CPU: 1 %
      : Total CPU cycles used: 147340
 thread_analyzer     : STACK: unused 544 usage 480 / 1024 (46 %); CPU: 1 %
      : Total CPU cycles used: 169843
 sysworkq            : STACK: unused 208 usage 304 / 512 (59 %); CPU: 0 %
      : Total CPU cycles used: 1
 idle                : STACK: unused 208 usage 48 / 256 (18 %); CPU: 91 %
      : Total CPU cycles used: 8529054
 main                : STACK: unused 144 usage 368 / 512 (71 %); CPU: 1 %
      : Total CPU cycles used: 107724
 ISR0                : STACK: unused 192 usage 832 / 1024 (81 %)

Below is the observed stack usages now under Broadcast ISO
audio application on nrf5340_audio_dk_nrf5340_cpunet:
Thread analyze:
 0x21001968          : STACK: unused 248 usage 264 / 512 (51 %); CPU: 2 %
      : Total CPU cycles used: 622241
 HCI rpmsg TX        : STACK: unused 240 usage 912 / 1152 (79 %); CPU: 2 %
      : Total CPU cycles used: 504720
 BT RX               : STACK: unused 464 usage 176 / 640 (27 %); CPU: 0 %
      : Total CPU cycles used: 3
 BT RX pri           : STACK: unused 200 usage 184 / 384 (47 %); CPU: 0 %
      : Total CPU cycles used: 212981
 thread_analyzer     : STACK: unused 544 usage 480 / 1024 (46 %); CPU: 1 %
      : Total CPU cycles used: 419593
 sysworkq            : STACK: unused 336 usage 176 / 512 (34 %); CPU: 0 %
      : Total CPU cycles used: 1
 idle                : STACK: unused 208 usage 48 / 256 (18 %); CPU: 91 %
      : Total CPU cycles used: 22065557
 main                : STACK: unused 144 usage 368 / 512 (71 %); CPU: 1 %
      : Total CPU cycles used: 388285
 ISR0                : STACK: unused 304 usage 720 / 1024 (70 %)

Relates to commit 3f7fa919 ("samples: Bluetooth: hci_rpmsg:
Fix RAM overflow building for DF feature").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>