staging: rtl8712: Remove NULL comparison

Remove NULL comparison by using '!' operator. Problem found using
checkpatch.pl
CHECK: Comparison to NULL could be written "!padapter->halpriv.hal_bus_init"

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>