dahdi: Do not set rxbufpolicy when opening dahdi net device.

This fixes a regression introduced in 2.7.0 in commit
(da8b96d72575a6f819d6e465ab93700a5f9f633f "dahdi: Remove unused 'rxbufpolicy'
and 'rxdisable' from dahdi_chan.") when CONFIG_DAHDI_NET is defined in
include/dahdi/dahdi_config.h.

rxbufpolicy was always hardcoded to immediate policy and was removed from the
channel structure. There is no longer any need to set it in dahdi_net_open.

Reported-by: Dave Fullerton <dfullertasterisk@shorelinecontainer.com>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>