rt2x00: Revise irqmask locking for PCI devices

The PCI device irqmask is locked by a spin_lock. Currently
spin_lock_irqsave is used everywhere. To reduce the locking overhead
replace spin_lock_irqsave in hard irq context with spin_lock and in
soft irq context with spin_lock_irq.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>