net: atlantic: fix potential error handling

commit 380ec5b9 upstream.

Code inspection found that in case of mapping error we do return current
'ret' value. But beside error, it is used to count number of descriptors
allocated for the packet. In that case map_skb function could return '1'.

Changing it to return zero (number of mapped descriptors for skb)

Fixes: 018423e9

 ("net: ethernet: aquantia: Add ring support code")
Signed-off-by: Pavel Belous <pbelous@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: Dmitry Bogdanov <dbogdanov@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>