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 <[e-mail]>
CC: Jay Vosburgh <[e-mail]>
CC: Veaceslav Falico <[e-mail]>
CC: Andy Gospodarek <[e-mail]>
CC: David S. Miller <[e-mail]>

Signed-off-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/bonding/bond_options.c

···
 	return &bond_opts[option];
 }
 
-int bond_option_mode_set(struct bonding *bond, const struct bond_opt_value *newval)
+static int bond_option_mode_set(struct bonding *bond,
+				const struct bond_opt_value *newval)
 {
 	if (!bond_mode_uses_arp(newval->value) && bond->params.arp_interval) {
 		pr_info("%s: %s mode is incompatible with arp monitoring, start mii monitoring\n",