drivers/usb/host: remove leftover release_mem_region

When converting this driver to devm_ioremap_resource, the removal of this now
unneeded function has been forgotten.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>