rpmsg: glink: spi: Rework glink spi bus voting logic

The spi suspend callback should be used when a driver would like to
suspend the bus, it does not act as a voting mechanism. Instead of the
suspend callback, GLINK should respond to the SUSPEND_EVENT with EBUSY
if there are transactions in flight. Change the activity flag to a
counter to keep track of the different active threads.

Before sending any data on the spi bus, GLINK should call the resume
callback. Increment the activity count on each thread before command
processing.

Change-Id: Iefbbac78bdd9d171bdfd2fdb3a339f040eb4347a
Signed-off-by: Chris Lew <clew@codeaurora.org>