drm: aspeed: Fix GENMASK misuse

Arguments to GENMASK should be msb >= lsb.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20200222235152.242816-1-megous@megous.com