Rename command defines to match names used in the datasheet, in order to

Rename command defines to match names used in the datasheet, in order to
make maintaining this driver from the documentation easier in the future.
This is a mostly mechanical change.

In uslcom_param(), move the zeroing of the final two fields of the
flowctrl structure outside of the "if CRTSCTS" section - not only were
they being zeroed in both the clauses, but these two fields have nothing
to do with hardware flow control anyway.