DaVinci EMAC: Fix davinci_eth_gigabit_enable

Enabling the gigabit was overwriting the
previous configuration by setting up only GIGAFORCE and
GIG bits of MAC control register.
Modified to retain previous configuration while
gigabit enabling.

Signed-off-by: Prakash PM <prakash.pm@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>