wifi: iwlwifi: pcie: rename CAUSE macro

At least mips64 has ist own CAUSE macro, so rename ours to
IWL_CAUSE to fix build issues.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Fixes: c1918196 ("iwlwifi: pcie: simplify MSI-X cause mapping")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220523220300.682be2029361.I283200b18da589a975a284073dca8ed001ee107a@changeid