[WAN]: fix section mismatch warning in sbni

In latest -mm sbni gives following warning: WARNING:
drivers/net/wan/sbni.o - Section mismatch: reference to \ .init.data:
from .text between 'init_module' (at offset 0x14ef) and \
'cleanup_module'

The warning is caused by init_module() calling a function declared
__init.  Declare init_module() __init too to fix warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>