Some ATI-based PowerBooks use the string 'mnca' in the backlight controller
device tree entry, so account for this and don't use nVidia when it's not an
nVidia device.
MFC after: 3 weeks
shifts into the sign bit. Instead use (1U << 31) which gets the
expected result.
This fix is not ideal as it assumes a 32 bit int, but does fix the issue
for most cases.
A similar change was made in OpenBSD.
Discussed with: -arch, rdivacky
Reviewed by: cperciva
