exynos4210: workaround UBSAN compilation error

gcc 5.4.0-6ubuntu1~16.04.5 build with UBSAN enabled error:

  CC      hw/display/exynos4210_fimd.o
/home/petmay01/linaro/qemu-for-merges/hw/display/exynos4210_fimd.c: In
function ‘fimd_get_buffer_id’:
/home/petmay01/linaro/qemu-for-merges/hw/display/exynos4210_fimd.c:1105:5:
error: case label does not reduce to an integer constant
     case FIMD_WINCON_BUF2_STAT:

Because FIMD_WINCON_BUF2_STAT case contains an integer
overflow, use U suffix to get the unsigned type.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180116151152.4040-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>