drivers: wifi: simplelink: socket() should return non-zero fds

simplelink_socket() is not returning non-zero socket file descriptors
when it succeeds. This leads to socket() to always return socket fd 0,
even when the network processor returns a non-zero fd.

This commit fixes simplelink_socket() to return the correct socket fd
when it succeeds in getting one from the network processor.

Fixes #12650

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>