hw/ide/ahci.h: Avoid shifting left into sign bit

Add 'U' suffixes to avoid undefined behaviour shifting left into
the signed bit of a signed integer type. Clang's sanitizer will
warn about this:

 hw/ide/ahci.c:1210:27: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
