Bluetooth: drivers: Add names to Silabs EFR32 threads

This makes thread analysis output nicer, e.g.:

uart:~$ kernel thread list
Scheduler: 25953 since last call
Threads:
 0x20002380 EFR32 HCI RX
        options: 0x0, priority: -10 timeout: 0
        state: pending, entry: 0x80228fd
        stack size 512, unused 216, usage 296 / 512 (57 %)

 0x20002478 EFR32 LL
        options: 0x0, priority: -16 timeout: 0
        state: pending, entry: 0x8022921
        stack size 1024, unused 296, usage 728 / 1024 (71 %)

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>