usb: xhci: add helper to read PORTSC register

usb: xhci: add helper to read PORTSC register

Add a dedicated helper function to read the USB Port Status and Control
(PORTSC) register. This complements xhci_portsc_writel() and improves code
clarity by providing a clear counterpart for reading the register.

Suggested-by: Peter Chen <peter.chen@kernel.org>
Reviewed-by: Peter Chen <peter.chen@kerne.org>
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/20251119142417.2820519-7-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>