staging: greybus: operation: add generic timeout support

Add a struct timer_list to struct gb_operation and use that to implement
generic operation timeouts.

This simplifies the synchronous operation handling somewhat while also
providing a generic timeout mechanism that drivers can use for
asynchronous operations.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>