USB: Let usb_sg_init to set transfer_buffer more often

This fix permits the "new" usbmon to access usb-storage's data buffer
without DMA remapping tricks. It should be compatible with PIO controllers
and not add any new crashes. Note that from now on PIO controllers and
usbmon are uniform in their access pattern and if one crashes then
the other will too. Hopefuly neither does.

As a side effect, we get rid for #ifdefs, which were a little ugly.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>