xhci: refactor xhci_urb_enqueue a bit with minor changes

make the local ep_state variable a pointer to the actual ring ep_state.
This allows us to read fresh ep_state values every time, will be useful
later.

Also move the streams check out from bulk only case. Even if only
bulk tranfers can use streams we shouldn't continue if those flags
are set. Main reason for this change is really code readability and
grouping functionality

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