USB: fix oops in usb_sg_init()

This patch (as1401) fixes a bug in usb_sg_init() that can cause an
invalid pointer dereference.  An inner loop reuses some local variables
in an unsafe manner, so new variables are introduced.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>