usb: phy: Improve Kconfig help for CONFIG_USB_PHY

The previous text confused users by not describing the very common
(e.g. x86 PC) sitations where no PHY driver is necessary.

Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>