Fix typo: WifiInterface -> WiFiInterface and add missing return.
Fix typo: WifiInterface -> WiFiInterface and add missing return.

Default WiFiInterface did not even build. And when it was fixed to build,
it was always returning NULL.

kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,mbedmicro/mbed,betzw/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed