drivers/net/vsc9953: Fix bug when enabling a port

When a port is enabled at init time, the initializing function
touches more bits than necessary to enable a port (also touches
reserved bits and default bit values). This patch fixes this issue
by changing the value of the define used to enable the port and
assures that no other bits are changes by replacing out_le32()
with setbits_le32().

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@freescale.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: York Sun <yorksun@freescale.com>