usb_serial: Fix remaining ref count/lock bugs

This fixes
- locking bug that was hidden by ecc2e05e
- Regression #13821
- Spurious warning when closing and blocking for data write out

With these changes my PL2303 always ends up as ttyUSB0 when it should and
the module refcounts stay correct.

I'll do a more wholesale split & tidy of _open in the next release or two
as we get a standard tty_port_open and port->ops->init port->ops->shutdown
call backs.

Copy sent to Alan Stern and Carlos Mafra just to confirm it fixes all the
reports but it passes local testing with the same hardware as Alan Stern.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>