Added the new Linux device driver for the card detect LED and
added a LED service with an API to set/get the LED status.

Removed kfree calls to free the slot memory.  The use of
devm_ calls to create the memory removes the need to call
kfree.

This member does not need to be persisted as it can simply be returned
to the caller requesting the mailbox value.

There was no point in permanently allocating the buffers since their
lifetime was function scoped and thus they could be converted to
stack variables.

Let the compiler decide what to inline

The slave_busy status doesn't need to persist, so just use a stack variable
instead.

The work queue would have been leaked in the case where the GPIO request failed
because the error handling cases were in the wrong order.

 - Remove trailing whitespace
 - Wrap lines at 80 chars
 - Format else statements according to linux coding style

when there is an error stopping record push timer when a
session stopped event is received.

stopped.

GPIO requests were passing pointers to stack allocated strings under the
assumption that the label string contents would be copied to another
storage location during the request.  This assumption was incorrect.
The string pointer is just copied, so the pointer becomes unsafe to
follow once the requesting function returns or re-uses the string.