xhci: add helper to allocate command with input context

Add a xhci_alloc_command_with_ctx() helper to get rid of
one of the boolean parameters telling if a context should
be allocated with the command.

No functional changes, improves core readability

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