usb: xhci: improve debug messages during suspend

usb: xhci: improve debug messages during suspend

Improve debug output for suspend failures, particularly when the controller
handshake does not complete. This will become important as upcoming patches
significantly rework the resume path, making more detailed suspend-side
messages valuable for debugging.

Add an explicit check of the Save/Restore Error (SRE) flag after a
successful Save State (CSS) operation. The xHCI specification
(note in section 4.23.2) states:

 "After a Save or Restore State operation completes, the
  Save/Restore Error (SRE) flag in USBSTS should be checked to
  ensure the operation completed successfully."

Currently, the SRE error is only observed and warning is printed.
This patch does not introduce deeper error handling, as the correct
response is unclear and changes to suspend behavior may risk regressions
once the resume path is updated.

Additionally, simplify and clean up the suspend USBSTS CSS/SSS
handling code, improving readability and quirk handling for AMD
SNPS xHC controllers that occasionally do not clear the SSS bit.

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20260402131342.2628648-12-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>