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

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

These calls are executed from an event handler and were previously nonblocking,
but have no partial write handling. They send short packets so should be OK to
block, most likely the output buffer will be empty anyway.

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