ieee802154_shell: Only accept channels within expected range

Fixes the following issue:
	"In expression 1UL << chan - 1U, left shifting by more than 31
	bits has undefined behavior.  The shift amount, chan - 1U, is
	4294967295."

Coverity-CID: 167140
Jira: ZEP-2131
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>