am335x_get_usb_phy() retrieves the last probed USB phy. On the BeagleBone
with both PHYs enabled, this means that dependent on probe order, both
MUSB instances could end up with the same PHY.

Remove the global variable and have the MUSB driver parse the "phys"
property instead.

The cleaner way to achieve this would be to migrate phy-am335x.c
and phy-am335x-control.c to the generic phy framework and have MUSB use
of_phy_get, alas, even Linux hasn't done this so far and we need
a short patch for master anyway, thus just do it the easy way.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>