isdn: avm: Fix string plus integer warning from Clang

A recent commit in Clang expanded the -Wstring-plus-int warning, showing
some odd behavior in this file.

drivers/isdn/hardware/avm/b1.c:426:30: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
                cinfo->version[j] = "\0\0" + 1;
                                    ~~~~~~~^~~
drivers/isdn/hardware/avm/b1.c:426:30: note: use array indexing to silence this warning
                cinfo->version[j] = "\0\0" + 1;
                                           ^
                                    &      [  ]
1 warning generated.

This is equivalent to just "\0". Nick pointed out that it is smarter to
use "" instead of "\0" because "" is used elsewhere in the kernel and
can be deduplicated at the linking stage.

Link: https://github.com/ClangBuiltLinux/linux/issues/309


Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>