nrf51: verify that UICR needs erasing before triggering an error about it

nrf51: verify that UICR needs erasing before triggering an error about it

If the UICR is already empty, there's no reason to return an error
just because it can't be erased again.  This happens, for example,
when flashing UICR from GDB after a "monitor nrf51 mass_erase".

Change-Id: Ia6d28c43189205fb5a7120b1c7312e45eb32edb7
Signed-off-by: Jim Paris <jim@jtan.com>
Reviewed-on: http://openocd.zylin.com/2363
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>