staging: comedi: amplc_pc236, amplc_pci236: get rid of bustype

The `bustype` member of `struct pc236_board` variables is initialized,
but never used.  Remove it along with the enumerated constants
associated with it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>