media: imx: imx7-media-csi: Don't double-enable the RxFIFO

The RxFIFO is enabled by imx7_csi_dmareq_rff_enable() in both
imx7_csi_init() and imx7_csi_enable(). Enabling it once is enough,
remove the former.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>