MFE: Add multi-frontend mutual exclusion

From: Darron Broad <darron@kewl.org>

This add frontend R/W mutual exclusion.
Prior to this point in time it was possible to open both
frontends simultaneously which an MFE card cannot support.

In order to stop this, a delayed open is performed which
has the following function:

 Return EBUSY after a configurable amount of time
if a frontend is unavailable due to the other being
in use.

 Only allow opening of a frontend if the kernel thread
of the other has stopped.

This solution was chosen to allow switching between
frontends to work as seamlessly as possible. When both
frontends are actually opened simultaneously then one
will only open, but if quick switching is performed
between one of many then the new open will succeed in
a clean fashion rather than interrupting a kernel
thread.

Priority: normal

Signed-off-by: Darron Broad <darron@kewl.org>

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