cpsw_mdio.c: Use correct reg in cpsw_mdio_get_alive

cpsw_mdio_get_alive reads the wrong register.
See page 2316 in SPRUH73Q AM335x TRM

Signed-off-by: Ulf Samuelsson <ulf@emagii.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>