iwlwifi: mvm: don't call << operator with a negative value

In https://bugzilla.kernel.org/show_bug.cgi?id=177341 Bob
reported a UBSAN WARNING on rs.c in iwldvm.
Fix the same bug in iwlmvm.

This because
	i = index - 1;
	for (mask = (1 << i); i >= 0; i--, mask >>= 1)

is unsafe: i could be negative and hence we can call <<
on a negative value.
This bug doesn't have any real impact since the condition
of the for loop will prevent any usage of mask.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=177341
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/rs.c

@@ -972,7 +972,9 @@ static u16 rs_get_adjacent_rate(struct iwl_mvm *mvm, u8 index, u16 rate_mask,
 
 		/* Find the previous rate that is in the rate mask */
 		i = index - 1;
-		for (mask = (1 << i); i >= 0; i--, mask >>= 1) {
+		if (i >= 0)
+			mask = BIT(i);
+		for (; i >= 0; i--, mask >>= 1) {
 			if (rate_mask & mask) {
 				low = i;
 				break;