orinoco: do not resgister NULL pm_notifier function

With DEBUG_NOTIFIERS it results in

[11330.890966] WARNING: at /home/bor/src/linux-git/kernel/notifier.c:88
notifier_call_chain+0x91/0xa0()
[11330.890977] Hardware name: PORTEGE 4000
[11330.890983] Invalid notifier called! ...

Without DEBUG_NOTIFIERS it most likely crashes on NULL pointer.

Signed-off-by: Andrey Borzenkov <arvidjaar@mail.ru>
Acked-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>