Error handling for register writes requires checking the error status of
every single write. This makes the code complex, or incorrect when the
checks are omitted. Simplify this by passing a pointer to an error code
to the imx290_write_reg() function, which allows writing multiple
registers in a row and only checking for errors at the end.

While at it, rename imx290_write_reg() to imx290_write() as there's
nothing else than registers to write, and rename imx290_read_reg()
accordingly.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>