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>
nrf51: fix checks for is_erased

is_erased can take the value 0 (no), 1 (yes), or -1 (unknown).
Checks like (!is_erased) don't do the right thing if it's -1.

Change-Id: I10ba32c99494ca803e0a7a1ba56fdd78184b96bb
Signed-off-by: Jim Paris <jim@jtan.com>
Reviewed-on: http://openocd.zylin.com/2366
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>