iwlwifi: rs: use const u16 for throughput tables

iwlwifi: rs: use const u16 for throughput tables

This makes the code a little bit longer as zero-extension
has to be done (mov vs. movzwl), but that's miniscule and
the space saving is significant, about 600 bytes in DVM
and 700 bytes in MVM, so the cache effect should be worth
the few bytes more code.

While at it, remove two spurious blank lines in variable
declaration blocks.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>