usb: xhci: simplify Isochronous Scheduling Threshold handling

usb: xhci: simplify Isochronous Scheduling Threshold handling

The IST is represented by bits 2:0, with bit 3 indicating the unit of
measurement, Frames or Microframes. Introduce xhci_ist_microframes(),
which returns the IST value in Microframes, simplifying the code and
reducing duplication.

Improve documentation in xhci-caps.h to clarify the IST register specifics,
including the unit conversion details. These change removes the need to
explain it each time the IST values is retrieved.

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/20251119142417.2820519-20-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>