Staging: xgifb: vb_util: Fixed sparse warning of bit truncation due to cast

Fixed the warning generated by sparse that 'cast truncates bits from
constant value' by typecasting unsigned values to u8 as their logical
operation is being performed with and stored in a u8 type variable.

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>