usb: xhci: guarantee that IMAN register is flushed

usb: xhci: guarantee that IMAN register is flushed

Add read call to guarantee that the write to the IMAN register has
been flushed.

xHCI specification 1.2, section 5.5.2.1, Note:
"Most systems have write buffers that minimize overhead, but this may
 require a read operation to guarantee that the write has been flushed
 from the posted buffer."

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20250515135621.335595-17-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>