usb-serial: back-port of pl2303.c from 2.6.24.1

usb-serial: back-port of pl2303.c from 2.6.24.1

I experienced major major data loss on a PL-2303 USB-serial converter
under 2.4.36, which I remedied by back-porting the pl2303.c from the
latest 2.6 kernel tree.

Here's a new patch, which is more complete than my previous one.
It's based on the 2.6.24.1.

There's a lot of trivial white-space changes and some things that have
been moved, which make the patch rather larger than it could be.  I
didn't include those changes before, but have now in order that the
driver be closer to the 2.6 driver.  It'll never be identical, of course.

Note, too, that the 2.6 driver (and thus the patched 2.4) includes a 1k
circular buffer which rather duplicates a buffer in the 2.4 usbserial.c;
2.6's usb-serial has had that buffer removed.  As the buffer resolves
loss of the occasional putchar (e.g. from n_tty's opost), it is
important and correct, even in 2.4.

Speaking as a user, I no longer see any problems with PL2303, and I
think this is okay to release.

Cc: Greg Kroah-Hartman <greg@kroah.com>