Kinetis: give a reasonable default for max_flash_prog_size

max_flash_prog_size euals to pflash_sector_size_bytes for most of devices.
There is no point setting max_flash_prog_size for devices without
FS_PROGRAM_SECTOR capability.
Check for zero sector_size to avoid div by zero exception in case of
device has FlexNVM but the driver does not define nvm_sector_size_bytes

Change-Id: Iaf4e007fb1ec3d24c373350410e4bebe504a4c3e
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/2958


Tested-by: jenkins
Reviewed-by: Thomas Schmid <thomas@rfranging.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Patrick Stewart <patstew@gmail.com>