media: staging/imx: remove devname string from imx_media_subdev

A separate string for the device name, for DEVNAME async match, was
never needed. Just assign the asd device name to the passed platform
device name pointer in imx_media_add_async_subdev().

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>