fluke-dmm: Make serial write calls block, and fix error handling.

fluke-dmm: Make serial write calls block, and fix error handling.

These calls were previously nonblocking, but have no partial write handling.
They are made from scan and acquisition_start contexts where they are free
to block.

Remove the SERIAL_NONBLOCK at open, which only applied during scan, since all
calls in the scan are now explicitly blocking.

Also fix error handling for these calls, which appears to have been kept
from a previous direct usage of write() on a serial port fd.