Exclude ch34x_driver.cpp from coverage

[Boernsman]

src/drivers/ch34x/ch34x_driver.cpp is the CH340 counterpart of
ftdi_driver.cpp: it is the code that actually talks to a device over libusb,
so the unit tests cannot reach it without hardware. ftdi_driver.cpp was already
excluded for that reason; this adds the CH340 one on the same grounds.

Merging the ch340 driver dropped the reported figure from 85.0% to 75.1%, against
a --fail-under-line 75 gate — it passed with 0.1 points to spare, and the next
chip family would have tripped it. The number was measuring how much untestable
transport code the tree contains rather than how well the testable code is
covered.

The scope comment is reworded to say the rule rather than list the files, since
every new chip family adds one more driver to it.