USB: Fix a bug on appledisplay.c regarding signedness

USB: Fix a bug on appledisplay.c regarding signedness

commit 37e9066b2f85480d99d3795373f5ef0b00ac1189 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>