gmc-mh-1x-2x: Make serial write calls block.

gmc-mh-1x-2x: Make serial write calls block.

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.

Fix error handling for some: serial_write can return any negative error code.