iwlwifi: mvm: rs: fix mimo delimiter in LQ cmd

mimo_delim was always set to 0 instead of pointing to
the first SISO entry after MIMO rates.
This can cause keep transmitting in MIMO even when we shouldn't.
For example when the peer is requesting static SMPS.

Cc: <stable@vger.kernel.org> [3.14]
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>