net: realtek: r8169: remove redundant assignment to giga_ctrl

The variable giga_ctrl is being assigned to zero however this is
never read and hence the assignment is redundant, so remove it.
Cleans up clang warning:

drivers/net/ethernet/realtek/r8169.c:1978:3: warning: Value stored
to 'giga_ctrl' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>