dell-wmi: Clean up hotkey table size check

Checking the table for a minimum size of 7 bytes makes no sense: any valid
hotkey table has a size that's a multiple of 4.

Clean this up: replace the hardcoded header length with a sizeof and
change the check to ignore an empty hotkey table.  The only behavior
change is that a 7-byte table (which is nonsensical) will now be
treated as absent instead of as valid but empty.

Reported-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>