Fix uint8_t comparisons with negative values

Fix the following warnings:
/src/qemu/hw/ide/core.c: In function `ide_drive_pio_post_load':
/src/qemu/hw/ide/core.c:2767: warning: comparison is always false due to limited range of data type

/src/qemu/ui/vnc-enc-tight.c: In function `tight_detect_smooth_image':
/src/qemu/ui/vnc-enc-tight.c:284: warning: comparison is always true due to limited range of data type
/src/qemu/ui/vnc-enc-tight.c:297: warning: comparison is always true due to limited range of data type
/src/qemu/ui/vnc-enc-tight.c: In function `tight_encode_indexed_rect16':
/src/qemu/ui/vnc-enc-tight.c:456: warning: comparison is always false due to limited range of data type
/src/qemu/ui/vnc-enc-tight.c: In function `tight_encode_indexed_rect32':
/src/qemu/ui/vnc-enc-tight.c:457: warning: comparison is always false due to limited range of data type

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
