iwlwifi: mvm: rework sched scan channel configuration

The current sched scan channel configuration code
configures all the supported channels for scanning.

However, this can result in SYSASSERT in some cases,
when the configured channel is disabled.

Instead, configure only the channels given in the
req struct, and set the channel_count field appropriately.

While on it, change the code to use channel->hw_value
instead of recalculating the channel number.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>