iwlwifi: mvm: rs: fix max rate allowed if no rate is allowed

In case the rate mask for one of the modulations was zero the
max rate idx for that modulation was set to 32 (BITS_PER_LONG).
This is bad as it would later lead to an out of bounds access
to the expected tpt table. In most cases there was no real effect
as the expected tpt was set to 0 and this led to avoiding the
modulation effectively.
Fix the out of bounds access and explicitly skip the modulation
in case there's no rate allowed in it.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>