bcm63xx_enet: avoid uninitialized variable warning

gcc-7 and probably earlier versions get confused by this function
and print a harmless warning:

drivers/net/ethernet/broadcom/bcm63xx_enet.c: In function 'bcm_enet_open':
drivers/net/ethernet/broadcom/bcm63xx_enet.c:1130:3: error: 'phydev' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This adds an initialization for the 'phydev' variable when it is unused
and changes the check to test for that NULL pointer to make it clear
that we always pass a valid pointer here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>