ahci_softreset() used to use ahci_tf_read() which reads D2H_REG area
to check for the Status register.  However, this area is zeroed on
initialization and not set by initial signature FIS.  Replace it with
ahci_check_status().

This bug prevented CLO code from being activated whenever BSY and/or
DRQ is set prior to softreset.  This fix makes
AHCI_FLAG_RESET_NEEDS_CLO flag redundant.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>