typo:assertion error when executing si.clock_2.r_divider = R_DIV_128

[evbaarle]

in file adafruit_si5351.py replace 6 by 7 in below fragment as R_DIV_128 has value 7:
@r_divider.setter
def r_divider(self, divider):
assert 0 <= divider <= 6

[sommersoft]

Fixed by #7