INT 13 calls return a status value via %ah, with CF set if %ah is
non-zero (indicating an error).  Our wrappers zero the whole of %ax if
CF is clear, to allow C code (which has no easy access to CF) to
simply test for a non-zero status to detect an error.

The current code assigns the returned status to a uint8_t, effectively
testing %al rather than %ah.  Fix by treating the returned status as a
uint16_t instead.

Signed-off-by: Michael Brown <mcb30@ipxe.org>