net: netcp: ethss: fix error in calling sgmii api with incorrect offset

On K2HK, sgmii module registers of slave 0 and 1 are mem
mapped to one contiguous block, while those of slave 2
and 3 are mapped to another contiguous block.  However,
on K2E and K2L, sgmii module registers of all slaves are
mem mapped to one contiguous block.  SGMII APIs expect
slave 0 sgmii base when API is invoked for slave 0 and 1,
and slave 2 sgmii base when invoked for other slaves.
Before this patch, slave 0 sgmii base is always passed to
sgmii API for K2E regardless which slave is the API invoked
for.  This patch fixes the problem.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>