drivers:misc: ti-st: flush UART upon fw failure

Upon failure to read firmware version from chip or upon failure in responses
to firmware download the UART needs to be flushed of its existing buffers so
that the UIM can restart UART properly.

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