iwlwifi: workaround interrupt handling no some platforms

This patch adds workaround for an interrupt related hardware bug on
some platforms.  (Apparently these platforms boot-up w/ INTX_DISABLED
set. -- JWL)

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>