mxs_ocotp: check for errors from the OTP controller after writing

The write operation may fail when trying to write to a locked area. In
this case the ERROR bit is set in the CTRL register. Check for that
condition and return an error.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>