media: dvb-core: Enhance shared multi-frontend support

Drivers for devices with multiple frontends which cannot be used
concurrently due to hardware limitations which enforce that restriction
by setting the mfe_shared field to 1 exhibit rather unfriendly behavior
towards applications: The unavailable frontend devices cannot be opened
at all, not even for read-only access to query information. Even worse,
any open call is blocked for 5 seconds by default.

Allow drivers for such devices to behave like regular busy frontend
devices instead, i.e. still allowing concurrent read access to the
unavailable frontend and denying concurrent write access with -EBUSY
without delay.

This patch does not alter the behavior of any existing driver to avoid
regressions. Driver developers who wish to take advantage of this must
ensure their driver can handle all read-only accesses to the unavailable
frontend, and indicate the capability by setting the mfe_shared field to
2 instead of 1.

Add a check to dvb-usb-init.c when automatically setting the mfe_shared
field that when a driver has already set the field to 2, it is not
overwritten.

Document the additional capability in the code comment about mfe_shared.

Link: https://lore.kernel.org/linux-media/trinity-22c77578-26b0-4867-9ff7-2668e5d22c64-1642799929896@3c-app-gmx-bap04


Signed-off-by: Robert Schlabbach <robert_s@gmx.net>
Tested-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>