iwlwifi: dvm: 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.

Undefined behaviour in drivers/net/wireless/intel/iwlwifi/dvm/rs.c:746:18

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>