flash: Fix flash write algorithm on pflash only devices.

This patch fix the slow flash write issue on pflash only devices.
The Family Reference Manual says:

For devices with FlexNVM: FlexRAM
For devices with program flash only: Programming acceleration RAM

So the acceleration RAM is available for the flash section command on
this device.

Change-Id: If6541a23a4457c5ed8858848a145f35cac63138b
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/509
Tested-by: jenkins
Reviewed-by: Tomas Frydrych <tf+openocd@r-finger.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/flash/nor/kinetis.c

@@ -320,7 +320,6 @@ static int kinetis_write(struct flash_bank *bank, uint8_t *buffer,
 		}
 	} else {
 		LOG_DEBUG("flash write into PFLASH @08%X", offset);
-		fallback = 1;
 	}