staging: mt7621-pci: IF statement expression comparing to NULL

Remove comparison to NULL in the if statement expression
to match the Linux Kernel coding style.
CHECK: Comparison to NULL could be written "res"

Signed-off-by: Branden Bonaby <brandonbonaby94@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>