xmc4xxx: Improve xmc4xxx_blank_check_memory() error handling

Clean up working area in case writing fails. Probably inherited from
armv7m_blank_check_memory(). Fix adapted from armv7m_checksum_memory().

Change-Id: I784bef481d1eba833ab6a9c34249fe9d43a16081
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3470


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>