Bugfix to allow programming of AVR chips with 256k

If isp.c line 129 is changed from:

	isp_hiaddr = 0;

to

	isp_hiaddr = 0xFF;

The usbasp can ben used to program larger chips, like those used in the Mega2560.
The fix is well documented here and in some online forums:
https://petervanhoyweghen.wordpress.com/2015/12/02/the-usbasp-and-atmega2560-mystery/