usb: xhci: cleanup IMOD register comments

usb: xhci: cleanup IMOD register comments

Patch does not contain any functional changes.

Add missing macro descriptions with specific bit definitions for each data
field and reordered them accordingly.

Remove "HW use only" from Interrupt Moderation Counter. xHCI Specification
1.2, section 5.5.2.2, states "This counter may be directly written by
software at any time to alter the interrupt rate."

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/20250515135621.335595-21-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>