bonding: Return nothing for not applicable values

From: Wagner Ferenc <wferi@niif.hu>

The previous code returned '\n' (that is, a single empty line)
from most files, with one exception (xmit_hash_policy), where
it returned 'NA\n'.  This patch consolidates each file to return
nothing at all if not applicable, not even a '\n'.

I find this behaviour more usual, more useful, more efficient
and shorter to code from both sides.

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