usb: make module xhci_hcd removable

Fixed regression. After commit 29e409f0 ("xhci: Allow xHCI drivers to
be built as separate modules") the module xhci_hcd became non-removable.
That behaviour is not expected and there're no notes about it in commit
message. The module should be removable as it blocks PM suspend/resume
functions (Debian Bug#666406).

Signed-off-by: Arthur Demchenkov <spinal.by@gmail.com>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>