b43: properly request pcmcia IRQ

PCMCIA needs an additional step to request the IRQ.

No need to add code to release the IRQ here, as that's done
automatically in pcmcia_disable_device().

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>