UBI: check if we are in RO mode in the erase routine

'do_sync_erase()' has to check whether we are in R/O mode before
erasing the PEB. This patch adds the check and while on it, adds an
assertion which validates the 'pnum' argument, as well as removes
a check which is always true because it has already been done
few lines before.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>