usb: chipidea: use generic map/unmap routines

We're one of the remaining drivers to map/unmap requests by hand. Switch
to using generic gadget routines for that instead.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>