iwlwifi: mvm: handle possible BIOS bug

In iwl_mvm_sar_get_ewrd_table() In case of a BIOS bug, n_profiles
might be 0 thus we need to return an error value.  Found by Klocwork.

Signed-off-by: Sharon Dvir <sharon.dvir@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>