iwlwifi: mvm: rs: fix a potential NULL deref

Found by klocwork analysis.
mvm could be NULL which may cause a NULL dereference
in a theoretical call flow

rs_fill_lq_cmd(mvm = NULL, ...)
rs_build_rates_table
rs_fill_rates_for_column
ucode_rate_from_rs_rate
IWL_ERR(mvm,...)

No real reason for passing NULL to rs_fill_lq_cmd so fix that.

Reported-by: Eytan Lifshitz <eytan.lifshitz@intel.com>
Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>