mISDN: avoid arch specific __builtin_return_address call

Not all architectures are able to call __builtin_return_address().
On ARM, the mISDN code produces this warning:

hardware/mISDN/w6692.c: In function 'w6692_dctrl':
hardware/mISDN/w6692.c:1181:75: warning: unsupported argument to '__builtin_return_address'
  pr_debug("%s: %s dev(%d) open from %p\n", card->name, __func__,
                                                                           ^
hardware/mISDN/mISDNipac.c: In function 'open_dchannel':
hardware/mISDN/mISDNipac.c:759:75: warning: unsupported argument to '__builtin_return_address'
  pr_debug("%s: %s dev(%d) open from %p\n", isac->name, __func__,
                                                                           ^

In a lot of cases, this is relatively easy to work around by
passing the value of __builtin_return_address(0) from the
callers into the functions that want it. One exception is
the indirect 'open' function call in struct isac_hw. While it
would be possible to fix this as well, this patch only addresses
the other callers properly and lets this one return the direct
parent function, which should be good enough.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>