Force the setting of bit 7 in the sysmouse packet byte 1 to be unsigned.

Clang complains about the shift of (1 << 7) into a int8_t changing the value:

warning: implicit conversion from 'int' to 'int8_t' (aka 'signed char') changes
value from 128 to -128 [-Wconstant-conversion]

Squash this warning by forcing clang to see it as an unsigned bit.

This seems odd, given that it's still a conversion of 128->-128, but I'm
guessing the explicit unsigned attribute notifies clang that sign really doesn't
matter in this case.

Reported by: Mark Millard <markmi AT dsl-only DOT net>
MFC after: 2 weeks