greybus: fw-mgmt: Add hooks to do mode-switch

This is the last step to required to finish the mode switch story.  That
is, call the hook provided by Interface layer to accomplish mode switch.

Tested on EVT 1.5 with gpbridge-test module.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Karthik Ravi Shankar <karthikrs@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>