handling of errors from `write` and `read` in `can.c`

[ukleinek]

@afa pointed out in https://github.com/pengutronix/microcom/pull/15#pullrequestreview-279354563 that with the signal handling in microcom errno cannot be EINTR when write or read returns with an error.

This allows some cleanup. (But I havn't understood the matter completely yet and also with some bits I understand today the signal handling isn't optimal and might see a redesign soon.)