media: aspeed: Use FIELD_GET to improve readability

Use the bitfield macro FIELD_GET, and GENMASK to do the shift and mask in
one go for reg values.

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>