USB: Fix a bug on appledisplay.c regarding signedness

brightness status is reported by the Apple Cinema Displays as an
'unsigned char' (u8) value, but the code used 'char' instead.

Note that he driver was developed on the PowerPC architecture,
where the two types are synonymous, which is not always the case.

Fixed that.  Otherwise the driver will interpret brightness
levels > 127 as negative, and fail to load.

Signed-off-by: pancho horrillo <pancho@pancho.name>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>