isdn/gigaset: correct CAPI voice connection encoding

Make the Gigaset CAPI driver select L2_VOICE (AT^SBPR=2) as the
layer 2 encoding for transparent connections, like the ISDN4Linux
variant.  L2_BITSYNC (AT^SBPR=0) mutes internal connections and
distorts external ones.

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