vfio-mdev/samples: Use u8 instead of char for handle functions

Clang warns:

samples/vfio-mdev/mtty.c:592:39: warning: implicit conversion from 'int'
to 'char' changes value from 162 to -94 [-Wconstant-conversion]
                *buf = UART_MSR_DSR | UART_MSR_DDSR | UART_MSR_DCD;
                     ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
1 warning generated.

Turns out that all uses of buf in this function ultimately end up stored
or cast to an unsigned type. Just use u8, which has the same number of
bits but can store this larger number so Clang no longer warns.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>