dp8393x: Have dp8393x_receive() return the packet size

dp8393x: Have dp8393x_receive() return the packet size

This function re-uses its 'size' argument as a scratch variable.
Instead, declare a local 'size' variable for that purpose so that the
function result doesn't get messed up.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit 9e3cd456d85ad45e72bdba99203302342ce29b3b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>