usb: of: fix build breakage caused by recent patches

commit 052a11d1 (usb: phy: make PHY driver selection
possible by controller drivers) changed the rules
on how drivers/usb/phy/of.c would be compiled and
failed to update include/linux/usb/of.h accordingly.

Because of that, we can fall into situations where
of_usb_get_phy_mode() is redefined. In order to fix
the error, we update the IS_ENABLED() check in
include/linux/usb/of.h to reflect the condition
where of.c is built.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Felipe Balbi <balbi@ti.com>