samples: net: wifi: boards: rdrw612bga: Modify CONFIG_POSIX_MAX_FDS config.

[Reproduce issue]
1. Connect In-STA to Ex-AP
   wifi connect WiFi_STA_HE_1.1.8 1234567890 3 2 2

2. Start APUT with ACS using below commands
   wifi ap enable nxp-AP 161 1234567890 3

3. Connect Ex-STA to APUT and verify association using ping traffic

4. Now start two TCP server
  zperf tcp download -I ml 5001 192.168.0.252
  zperf tcp download -I up 6002 192.168.10.1

Observed behavior:
=============

Second TCP server fails to start

[Analysis]
1. After starting uap and a tcp server,
   there is only one allocatable fd resource
   left in the fdtable.
2. When two TCP clients try to establish a connection
   with the server at the same time, the second client
   cannot be assigned an fd, resulting in a failure.

[Fix]
Increase the maximum number of open file descriptors.

Signed-off-by: Qiankun Li <qiankun.li@nxp.com>