iwlwifi: mvm: fix comparison of u32 variable with less than zero

The comparison of the u32 variable wgds_tbl_idx with less than zero is
always going to be false because it is unsigned.  Fix this by making
wgds_tbl_idx a plain signed int.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: 4fd445a2 ("iwlwifi: mvm: Add log information about SAR status")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>