gianfar: Fix potential soft reset race

SOFT_RESET must be asserted for at least 3 TX clocks in order for it to work
properly.  The syncs in the gfar_write() commands have been hiding this, but
we need to guarantee it.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>