iwlwifi: mei: avoid -Wpointer-arith and -Wcast-qual warnings

These cause extra warnings (at least with W=3), avoid them.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220130115024.59d00c6e5f09.I331bf18af0b80b5cc41b379ff0cc1b26a89dd915@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>