NET: fec: only enable napi if we are successful

If napi is left enabled after a failed attempt to bring the interface
up, we BUG:

fec 2188000.ethernet eth0: no PHY, assuming direct connection to switch
libphy: PHY fixed-0:00 not found
fec 2188000.ethernet eth0: could not attach to PHY
------------[ cut here ]------------
kernel BUG at include/linux/netdevice.h:502!
Internal error: Oops - BUG: 0 [#1] SMP ARM
...
PC is at fec_enet_open+0x4d0/0x500
LR is at __dev_open+0xa4/0xfc

Only enable napi after we are past all the failure paths.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>