USB: make usb_buffer_map_sg consistent with doc

usb_buffer_map_sg should return negative on error according to
its documentation. But dma_map_sg returns 0 on error. Take this
into account and return -ENOMEM in such situation.

While at it, return -EINVAL instead of -1 when wrong input is
passed in.

If this wasn't done, usb_sg_* operations used after usb_sg_init
which returned 0 may cause oopses/deadlocks since we don't init
structures/entries, esp. completion and status entry.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>