drm/panel: declare variable as __be16

Declare the temp variable as __be16 to address the following sparse
warning:
drivers/gpu/drm/panel/panel-lg-lg4573.c:45:20: warning: incorrect type in initializer (different base types)
drivers/gpu/drm/panel/panel-lg-lg4573.c:45:20:    expected unsigned short [unsigned] [usertype] temp
drivers/gpu/drm/panel/panel-lg-lg4573.c:45:20:    got restricted __be16 [usertype] <noident>

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191230195609.12386-1-wambui.karugax@gmail.com