hw/ide/ahci.c: Fix shift left into sign bit

Avoid undefined behaviour from shifting left into the sign bit:

hw/ide/ahci.c:551:36: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'

(Unfortunately C's promotion rules mean that in the expression
"some_uint8_t_variable << 24" the LHS gets promoted to signed
int before shifting.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: John Snow <jsnow@redhat.com>
