media: i2c: imx290: Switch set_hmax to use imx290_write_buffered_reg

imx290_set_hmax was using two independent writes to set up hmax,
when all other multi-register writes were using imx290_write_buffered_reg
which claims the group hold first.

Switch imx290_set_hmax to using imx290_write_buffered_reg too.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>