stm class: Factor out default framing protocol

The STP framing pattern that the stm class implicitly applies to the
data payload is, in fact, a protocol. This patch moves the relevant code
out of the stm core into its own driver module.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>