flash/nor/kinetis: use target_get_working_area_avail()

flash/nor/kinetis: use target_get_working_area_avail()

Since e22c6484eaedd56d71 the trial/error allocation shows
"not enough working area available(requested 2048)" message
on klx.cfg. The message is not clear if it means a problem or not.

Replace with new style allocation using target_get_working_area_avail()

Change-Id: I87fe1e38248fcac29982b72aaba12217a0552f38
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5752
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>