spi_butterfly null ptr dereference on unload module

For the record ... at first impression this
feels like a regression in another part of
the kernel than the driver you indicate.
I used to do that load/unload semi-regularly,
with no errors appearing.

However, I can't do any real testing on this,
now, since I no longer have parport-capable
hardware to which I can hook up one of these adapters ... and thus collect more info about where the bit-rot has rooted.

- Dave