isdn/gigaset: correct bas_gigaset rx buffer handling

In transparent data reception, avoid a NULL pointer dereference
in case an skbuff cannot be allocated, remove an inappropriate
call to the HDLC flush routine, and correct the accounting of
received bytes for continued buffers.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
CC: stable <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>