i2c_bit_add_bus can call the pre_xfer function, which expects the func
pointer to be set. Pass in func to the port creation logic so that it is
set before i2c_bit_add_bus.

See https://bugs.freedesktop.org/show_bug.cgi?id=68456Reported-by: Hans-Peter Deifel <hpdeifel@gmx.de>
Tested-by: Hans-Peter Deifel <hpdeifel@gmx.de>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>