iwlwifi: mvm: configure phy_ctxt with min_def

Configure the phy context to the minimum required
bandwidth, given by ctx->min_def.

Tuning to a narrower bandwidth should reduce the
noise level and consume less power.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>