drivers:misc: ti-st: reinit completion before send

download firmware behaves differently at different times, when logs are
enabled and the system is loaded, the wait_for_completion is able to wait for
every send, However during other times the wait does not happen.

So, for reliability reinitializing the completion before every send, makes
sure the wait happens for every send.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>