greybus: loopback: warn user if kfifo cannot log all data

The depth of the kfifo used to log the latency data for user-space can be
moved upwards or downward by way of a module parameter. The user may still
specify a test set that's larger than the number of kfifo elements we have
available. If the user specifies more iterations than can be logged give a
warning as feedback and continue with the test.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>