staging: rtl8712: remove unnecessary NULL check

List wlan_network is built properly with init_list_head and
list_add_tail which cancels out its probability of being NULL because of
the precence of list_head at the top of this structure. Hence, the NULL
test can be omitted.

Signed-off-by: Himadri Pandya <himadri18.07@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>