net: keystone_net: remove redundant code from keystone_net.c

Remove unused tx_send_loop variable.

Removes duplicated get_link_status() call from the
keystone2_eth_send_packet().

The emac_gigabit_enable() is called at opening Ethernet and there is no
need to enable it on sending each packet. So remove that call
from keystone2_eth_send_packet() as well.

The calling of power/clock up functions are mostly the responsibility
of SoC/board code, so move these functions to appropriate place.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>