bonding: fix bond_option_mode_set warning

During the conversion to "static" functions this one got left out, only its
prototype was converted, thus resulting in:
drivers/net/bonding//bond_options.c:674:5: warning: symbol
'bond_option_mode_set' was not declared. Should it be static?

Fix it by making it static and also break the line in two as it was too
long.

CC: Stephen Hemminger <stephen@networkplumber.org>
CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: David S. Miller <davem@davemloft.net>

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>