tsec: Fix a bug in soft-resetting

SOFT_RESET must be asserted for at least 3 TX clocks.  Usually, that's about 30
clock cycles, so it's been mostly working.  But we had no guarantee, and at
slower bitrates, it's just over a microsecond (over 1000 clock cycles).  This
enforces a 2 microsecond gap between assertion and deassertion.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>