continue 4bc68c9 Fix icebox_vlog for up5k

[majbthrd]

I was confronted with this when attempting to use icebox_vlog with a up5k image:

Traceback (most recent call last):
  File "/home/user/icestorm/icebox/./icebox_vlog.py", line 537, in <module>
    text_func.append("  .SHIFTREG_DIV_MODE(1'b%s)," % get_pll_bit(pllinfo, "SHIFTREG_DIV_MODE"))
  File "/home/user/icestorm/icebox/./icebox_vlog.py", line 186, in get_pll_bit
    bit = pllinfo[name]
KeyError: 'SHIFTREG_DIV_MODE'

The fault looks suspiciously like another fault already fixed by previous PR #263, and so I've applied the same @daveshah1 correction.