iwlwifi: mvm: rs: refactor to use rs_rate

Introduce rs_rate which represents a rate. Use this structure
instead of iwl_scale_tbl_info where we're dealing with a single
rate.
This avoids allocating the big iwl_scale_tbl_info structure
on the stack in several cases like converting to ucode rate
format or from ucode rate format.

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