usb: musb: fix Kconfig regression

A recent bug fix I did that was marked for stable backports
introduced a slightly wrong dependency on CONFIG_OMAP_CONTROL_PHY.

I was missing the fact that the PHY driver already stubs out the
omap_control_usb_set_mode, and we only need to add a dependency
to prevent the musb-omap2430 driver from being built-in when
the phy driver is a loadable module, but we should not prevent it
from being built altogether when the phy driver is disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: ca784be3 ("usb: start using the control module driver")
Cc: <stable@vger.kernel.org> # v3.9+
Acked-by: Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>