drm/tiny: fix sparse warning: incorrect type in assignment (different base types)

This fixes the following sparse warning:

drivers/gpu/drm/tiny/ili9486.c:61:16: sparse: sparse: incorrect type in assignment (different base types)
drivers/gpu/drm/tiny/ili9486.c:61:16: sparse:    expected unsigned short [usertype]
drivers/gpu/drm/tiny/ili9486.c:61:16: sparse:    got restricted __be16 [usertype]
drivers/gpu/drm/tiny/ili9486.c:71:32: sparse: sparse: incorrect type in assignment (different base types)
drivers/gpu/drm/tiny/ili9486.c:71:32: sparse:    expected unsigned short [usertype]
drivers/gpu/drm/tiny/ili9486.c:71:32: sparse:    got restricted __be16 [usertype]

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Kamlesh Gurudasani <kamlesh.gurudasani@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1583684084-4694-1-git-send-email-kamlesh.gurudasani@gmail.com