Revert "serial: set the errno if and only if in thread context"

This reverts commit 07bd09fc0fc256e5c53ad3c6157802fd94c9910c.
The validation of the context is done with in `rt_set_errno`. So it's
safe to call it here.
Merge pull request #187 from grissiom/serial-fix

Revert "serial: set the errno if and only if in thread context"