iwlagn: improve RXON checking

The current RXON checking doesn't verify that
the channel is valid (or at least non-zero),
so add that. Also, add a WARN() so we get a
stacktrace, and capture a bitmask of errors
in order to capture all necessary information
in the warning itself (in case the previous
messages are snipped off.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>