USB: ftdi_sio: Keep going when write errors are encountered.

The use of urb->actual_length to update tx_outstanding_bytes
implicitly assumes that the number of bytes actually written is the
same as the number of bytes we tried to write.  On error that
assumption is violated so just use transfer_buffer_length the number
of bytes we intended to write to the device.

If an error occurs we need to fall through and call
usb_serial_port_softint to wake up processes waiting in
tty_wait_until_sent.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>