farsync: add module_put to error path in fst_open()

The fst_open() function, after a successful try_module_get() may return
an error code if hdlc_open() returns it.  However, it does not put the
module on this error path.

This patch adds the necessary module_put() call.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Pavel Shved <shved@ispras.ru>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>