media: i2c: Kconfig: Make MAX9271 a module

With the introduction of the RDACM21 camera module support in commit
a59f853b ("media: i2c: Add driver for RDACM21 camera module") the
symbols defined by the max9271 library were exported twice if multiple
users of the library were compiled in at the same time.

In example:
WARNING: modpost: drivers/media/i2c/rdacm21-camera_module:
'max9271_set_serial_link' exported twice. Previous export was in
drivers/media/i2c/rdacm20-camera_module.ko

Fix this by making the max9271 file a module and have the driver
using its functions select it.

Fixes: a59f853b ("media: i2c: Add driver for RDACM21 camera module")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Suggested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>