ibmvnic: Fixup atomic API usage

Replace a couple of modifications of an atomic followed
by a read of the atomic, which is no longer atomic, to
use atomic_XX_return variants to avoid race conditions.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>