samples: sockets: Configure correct number of FDs for echo_server
samples: sockets: Configure correct number of FDs for echo_server

echo_server can use up to 6 sockets therefore it needs more file
descriptors than the maximum default value specified.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

finikorg/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,punitvara/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr