iwlwifi: mvm: rs: refactor building the LQ command

Simplify the code a bit more by extracting the rates table
building logic into a separate function and handle setting
a fixed rate for debug in a separate flow.
Also avoid using and saving ucode rate format in different
places. Instead use rs_rate struct and convert to ucode format
only when filling the rates table in the LQ command.

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