usb: xhci: move device slot enabling register write

usb: xhci: move device slot enabling register write

Refactor the setting of the Number of Device Slots Enabled field into a
separate function, relocating it to xhci_init().

The xHCI driver consistently sets the number of enabled device slots to the
maximum value. The new function is named to reflect this behavior.

Remove the "// " prefix from trace messages, as it is unnecessary and
distracting.

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/20250515135621.335595-5-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>