USB: serial: ch341: reinitialize chip on reconfiguration

Changing the LCR register after initialization does not seem to be reliable
on all chips (particularly not on CH341A). Restructure initialization and
configuration to always reinit the chip on configuration changes instead and
pass the LCR register value directly to the initialization command.

(Note that baud rates above 500kbaud are incorrect, but they're incorrect in
the same way both before and after this patch at least on the CH340G. Fixing
this isn't a priority as higher baud rates don't seem that reliable anyway.)

Cleaned-up version of a patch by Grigori Goronzy

Signed-off-by: Aidan Thornton <makosoft@gmail.com>
Reviewed-by: Grigori Goronzy <greg@chown.ath.cx>
Signed-off-by: Johan Hovold <johan@kernel.org>