[media] mem2mem_testdev: improve field handling

try_fmt should just set field to NONE and not return an error if
a different field was passed.

buf_prepare should check if the field passed in from userspace has a
supported field value. At the moment only NONE is supported and ANY
is mapped to NONE.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>