iwlwifi: mvm: use NULL instead of ERR_PTR when parsing wowlan status

We anyway don't differentiate between the errors so it is pointless,
returning NULL will be simpler in this case.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20220517120044.78a7651327bb.I77480de7c26db850680f96a3440fb6a1b45dd9d2@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>