xmc4xxx: Fix error propagation for erase_check callback

xmc4xxx: Fix error propagation for erase_check callback

If an error occurs during xmc4xxx_blank_check_memory() aka .erase_check,
it would break out of the loop over flash sectors and return ERROR_OK.

Instead return the error code so that tcl.c can notify the user.

Change-Id: Ie2c1b7933eef2b240b28f8a292634fbbf5b31706
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3425
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>