misc: rtsx: Find L1 PM Substates capability instead of hard-coding

Instead of hard-coding the location of the L1 PM Substates capability based
on the Device ID, search for it in the extended capabilities list.  This
works for any device, as long as it implements the L1 PM Substates
capability correctly, so it doesn't require maintenance as new devices are
added.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200721212336.1159079-5-helgaas@kernel.org


[ minor addition due to differences in my tree - gregkh]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>