iwlwifi: mvm: simplify iwl_mvm_get_wakeup_status() return

The return value in iwl_mvm_get_wakeup_status() is a bit unclear in
that it's not obvious that we don't leak fw_status in some cases.
Use fw_status directly with ERR_PTR() and return only it, that way
the compiler has a chance of proving that it's uninitialized (if it
ever is due to new changes.)

Additionally, this removes a smatch warning since smatch couldn't
figure out that fw_status can't, in fact, leak here.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>