stm class: dummy_stm: Add link callback for fault injection

stm class: dummy_stm: Add link callback for fault injection

STM device's link callback has the power to abort master/channel
assignment by returning a negative error code. Use this in dummy
stm device to optionally abort assigning certain channel IDs.
This is useful as fault injection into the stm class core, for
testing purposes.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit adcde635f5d6b77e9f11087af09c6f62da2bf108)