ahci: implement SCR_NOTIFICATION r/w

Make ahci_scr_read/write() handle SCR_NOTIFICATION if the controller
supports it.  Also, print "sntf" in the cap line if supported.

While at it, convert eight space into a tab in ahci_print_info().

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