change wsp.enable_min_nvols default value

[armenbod]

Currently I find that wsp.enable_min_nvols = 6 - I am not sure where this default is being set (either in oxasl / oxasl_enable).

This breaks

oxasl_enable/oxasl_enable/enable.py

Line 207 in a44d4f9

for i in range(wsp.enable_min_nvols, tdim+1, 1):

if number repeats i.e. tdim < 6, it will not loop.

It's best to keep default = None or 2. There is code already that would set it to 2 if it were None:

oxasl_enable/oxasl_enable/enable.py

Line 194 in a44d4f9

wsp.enable_min_nvols = wsp.ifnone("enable_min_nvols", 2)