ISDN: eicon: reduce stack size of sig_ind function

commit 27d807180ae0a9e50d90adf9b22573c21be904c2 upstream.

I noticed that this function uses a lot of kernel stack when the
"latent entropy" plugin is enabled:

drivers/isdn/hardware/eicon/message.c: In function 'sig_ind':
drivers/isdn/hardware/eicon/message.c:6113:1: error: the frame size of 1168 bytes is larger than 1152 bytes [-Werror=frame-larger-than=]

We currently don't warn about this, as we raise the warning limit
to 2048 bytes in mainline, but I'd like to lower that limit again
in the future, and this function can easily be changed to be more
efficient and avoid that warning, by making some of its local
variables 'const'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>