iwlwifi: mvm: byte-swap constant instead of variable

Convention has it to byte-swap the constant instead of the variable
when doing bit checks. This also generates better code when the swap
is actually needed, since the constant can be swapped at compile-time.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>