rt61pci: fix NULL pointer dereference in config_lna_gain

commit deee0214def5d8a32b8112f11d9c2b1696e9c0cb upstream.

We can not pass NULL libconf->conf->channel to rt61pci_config() as it
is dereferenced unconditionally in rt61pci_config_lna_gain() subroutine.

Resolves:
https://bugzilla.kernel.org/show_bug.cgi?id=44361

Reported-and-tested-by: <dolohow@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>