bonding: Purely cosmetic: rename a local variable

From: Wagner Ferenc <wferi@niif.hu>

Code for rendering multivalue sysfs files occurs three times
in this module.  Rename 'buffer' to 'buf' in the first, for
the sake of consistency.

Signed-off-by: Ferenc Wagner <wferi@niif.hu>
Acked-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>