iwlwifi: disable interrupts before calling request_irq

Disable interrupts in the iwl4965 before calling request_irq() for
the case that the previous OS or the BIOS left a pending interrupt in
the chip. This behavior has been observed on some laptops such as T61
Thinkpads and Toshiba Portege R500

Signed-off-by: Jes Sorensen <jes@sgi.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>