sata_sil24: separate out sil24_do_softreset()

Separate out sil24_do_softreset() which takes @pmp as its last
argument.  This will be used to implement sil24_pmp_softreset().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>