serdev: document driver callbacks

Amend the driver-callback kerneldoc with calling context and expected
return values.

Note that this is based on the requirements and characteristics of the
tty-port controller implementation which receives data in workqueue
context and whose write_wakeup callback must not sleep.

Also note that while the receive_buf callback returns an integer, the
returned value is still expected to be non-negative (and no greater than
the buffer-size argument).

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>