b43: Don't disable IRQs in mac_suspend

This patch removes the IRQ-disable from mac_suspend.
The main advantage of this is to get rid of the IRQ-sync call in mac_suspend.
We need to remove the MAC suspend bit from the IRQ service mask, as otherwise
the IRQ handler would race with us.

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