usb/uhci: initialize sg_table properly

Commit 689d6eac ("USB: UHCI: add native scatter-gather support(v1))
added sg support to uhci but forgot to set the sg_table so this feature
remained unused.

Cc: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>