net/ibm/emac: wrong bit is used for STA control register write

STA control register has areas of mode and opcodes for opeations. 18 bit is
using for mode selection, where 0 is old MIO/MDIO access method and 1 is
indirect access mode. 19-20 bits are using for setting up read/write
operation(STA opcodes). In current state 'read' is set into old MIO/MDIO mode
with 19 bit and write operation is set into 18 bit which is mode selection,
not a write operation. To correlate write with read we set it into 20 bit.
All those bit operations are MSB 0 based.

Signed-off-by: Ivan Mikhaylov <ivan@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>