kinetis : Fix broken check for mass erase.

kinetis : Fix broken check for mass erase.

If the flash is not ready (MDM_STAT_FREADY is 0) then
dap_syssec_kinetis_mdmap() would act as if the MDM_STAT_SYSSEC bit was
set and erase the flash. Wait until MDM_STAT_FREADY is set before
checking the MDM_STAT_SYSSEC bit.

Change-Id: I5c3352f625599016625ed9be8787033f49bfacea
Signed-off-by: Per Ekman <pekenator@gmail.com>
Reviewed-on: http://openocd.zylin.com/1762
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>