apple-gmux: Hard Code max brightness for MMIO gmux

The data in the max brightness port for iMacs with MMIO gmux incorrectly
reports 0x03ff, but it should be 0xffff. As all other MMIO gmux models
have 0xffff, hard code this for all MMIO gmux's so they all have the
proper brightness range accessible.

Fixes: 0c18184d ("platform/x86: apple-gmux: support MMIO gmux on T2 Macs")
Reported-by: Karsten Leipold <poldi@dfn.de>
Signed-off-by: Orlando Chamberlain <orlandoch.dev@gmail.com>
Link: https://lore.kernel.org/r/20231017111444.19304-2-orlandoch.dev@gmail.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>