nRF51: Fix bug in Code memory size.

nRF51: Fix bug in Code memory size.

The code memory size was a bug and seemed to be working by
accident since there happened to be 256 pages in the device that was
tested on which corresponded to 256kb.

Also don't fail if memory size != expected memory size based on hwid
as this hwid is unstable and should be used only for debug/diagnostics.

Change-Id: I4e98f7498a36c53fc51783eddfdaba704d30e3ca
Signed-off-by: Michael Dietz <mjdietzx@gmail.com>
Reviewed-on: http://openocd.zylin.com/3510
Tested-by: jenkins
Reviewed-by: Jiří Pinkava <j-pi@seznam.cz>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>