uchcom: extend hardware support to version 0x30

This change adds support for a UBS<->RS232 adapter based on CH340 (or an
analogue) that I own. The device seems to have a newer internal version
(0x30) and the existing code incorrectly configures line control for it
resulting in garbled transmission. The changes are based on what I
learned in Linux drivers for the same hardware.

Additional changes:

• use UCHCOM_REG_LCR1 / UCHCOM_REG_LCR2 instead of explicit 0x18 and 0x25
• use NULL instead of 0 where a pointer is expected

Reviewed by: hselasky
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D15498