iwlwifi: Remove use of the deprecacted PTR_RET

This removes the use of the two deprecated calls to the
macro PTR_RET in iwl_mvm_get_regdomain and replaces them
both to PTR_ERR_OR_ZERO.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
[Commit message editing]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>