platform/x86: dell-wmi: add new dmi mapping for keycode 0xffff

This keycode is used by Dell as a no-op for keys that should have
no function.

This keycode is never triggered by a keypress in practice, rather
it is included from the 0xB2 DMI table at startup.

This prevents the following messages from being logged at startup on a
Dell Inspiron 5593:

    dell_wmi: firmware scancode 0x48 maps to unrecognized keycode 0xffff
    dell_wmi: firmware scancode 0x50 maps to unrecognized keycode 0xffff

as per this code comment:

   Log if we find an entry in the DMI table that we don't
   understand.  If this happens, we should figure out what
   the entry means and add it to bios_to_linux_keycode.

Signed-off-by: Y Paritcher <y.linux@paritcher.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Mario Limonciello <mario.limonciello@dell.com>