net: Fix build warning in sock_bindtodevice().

net/core/sock.c: In function 'sock_setsockopt':
net/core/sock.c:396: warning: 'index' may be used uninitialized in this function
net/core/sock.c:396: note: 'index' was declared here

GCC can't see that all paths initialize index, so just
set it to the default (0) and eliminate the specific
code block that handles the null device name string.

Signed-off-by: David S. Miller <davem@davemloft.net>