ar71xx: fix TL-MR10U usb power

MR10U is closer to MR3040 than WR703N. This changes also
enable USB power on GPIO18.

Signed-off-by: Cezary Jackiewicz <cezary.jackiewicz@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 36726