drm/mgag200: fix spelling mistake "expeced" -> "expected"

There is a spelling mistake in a drm_warn message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826084727.42703-1-colin.king@canonical.com