usb: xhci: implement USB Port Register Set struct

usb: xhci: implement USB Port Register Set struct

Previously, each port's 'addr' field pointed to the base of the Host
Controller USB Port Register Set, and specific registers were accessed
using macros such as (port->addr + PORTPMSC).

This patch replaces the raw '__le32 __iomem *addr' pointer with a typed
'struct xhci_port_regs __iomem *port_reg' pointer. With this change,
individual registers can be accessed directly through the structure
fields:

Before:
  port->addr
  port->addr + PORTPMSC
  port->addr + PORTLI
  port->addr + PORTHLPMC

After:
  port->port_reg->portsc
  port->port_reg->portpmsc
  port->port_reg->portli
  port->port_reg->porthlpmc

This improves code readability and makes register access more intuitive
by using named struct members instead of pointer arithmetic and macros.

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/20251119142417.2820519-9-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>