atxp1: Signed/unsigned char bug fix

vid_to_reg() can return -1 and char can be unsigned.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>