isdn/gigaset: correct CAPI DATA_B3 Delivery Confirmation

The Gigaset CAPI driver handled all DATA_B3_REQ messages as if the
Delivery Confirmation flag bit was set, delaying the emission of the
DATA_B3_CONF reply until the data was actually transmitted. Some
CAPI applications (notably Asterisk) aren't happy with that
behaviour. Change it to actually evaluate the Delivery Confirmation
flag as described the CAPI specification.

Impact: bugfix
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>