UBI: fix error handling in erase worker

Do not switch to read-only mode in case of -EINTR and some
other obvious cases. Switch to RO mode only when we do not
know what is the error.

Reported-by: Vinit Agnihotri <vinit.agnihotri@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>