nrf51: fix UICR erase

nrf51: fix UICR erase

nrf51_erase_page() checks for (sector->offset == NRF51_UICR_BASE) to
determine if the UICR should be erased.  However, sector->offset for
the UICR bank is set to 0 in nrf51_probe, so this code is never hit.
Attempting to erase UICR ends up erasing the first flash sector.

Use bank->base instead to determine if UICR is being erased.

Change-Id: Ie5df0f9732f23662085ae2b713d64968cd801472
Signed-off-by: Jim Paris <jim@jtan.com>
Reviewed-on: http://openocd.zylin.com/2362
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>