media: imx: imx7-media-csi: Simplify imx7_csi_rx_fifo_clear()

There's no need to read the CSICR1 register before each write in
imx7_csi_rx_fifo_clear(). Simplify the function.

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>