isci: Fix use of SATA soft reset state machine.

The driver SATA LUN reset function incorrectly sent an SRST deassert
FIS, which is unnecessary because the core initiates the entire SATA
soft reset state machine from the assert request.

Signed-off-by: Jeff Skirvin <jeffrey.d.skirvin@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>