net, phy, cpsw: fix gigabit register access

accessing a lan9303 switch with the cpsw driver results in wrong
speed detection, as the switch sets the BMSR_ERCAP in BMSR
register, and follow read of the MII_STAT1000 register fails, as
the switch does not support it. Current code did not check,
if a phy_read() fails ... fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>