wifi: iwlwifi: mld: rx: simplify channel handling

There's no need to have the channel in phy_data, this
code only supports one descriptor format so can use
it directly from there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20250424153620.409c2f4ecbcc.I1735cd773ed07ad9fae44bdfc46261fa9a8fd0e0@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>