iwlwifi: pcie: simplify MSI-X cause mapping

iwlwifi: pcie: simplify MSI-X cause mapping

We're currently manually encoding a calculation here since the HW
just maps all the bits of specific registers to specific offsets,
which led to the bug fixed here previously with the Bz SW_ERROR
interrupt.

Clean up the code to only know about the mapping offset (-16 or
16 depending on the register) to avoid such issues in the future.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20220517120044.19abe9a4d171.I934356911277f9b2a955808763f317986f69a461@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>