usb: phy: isp1301: work around tps65010 dependency

The isp1301-omap driver contains special hooks for the TPS65010
power management controller. It provides its own 'tps65010_set_vbus_draw'
wrapper in case that driver is not enabled through Kconfig, but
fails to handle the case where isp1301-omap is built-in but TPS65010
is a loadable module, which currently results in a link error:

drivers/built-in.o: In function `isp1301_set_power':
:(.text+0x14e188): undefined reference to `tps65010_set_vbus_draw'

This is a workaround to use the same trick as before also when
tps65010 is a module. Doing a proper fix would require much larger
changes to the driver that is not really worth it when the usb-phy
drivers are going to eventually get replaced with generic-phy
drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>