usb: phy: phy core cannot yet be a module

A lot of platform code calls into the usb phy core at the moment, which
does not work if it is built as a loadable module. This will hopefully
change when those platforms are all converted to DT based probing,
but for now, the easiest solution is to change it from "tristate"
to "bool".

This solves at least these ARM allmodconfig build errors:

arch/arm/mach-imx/built-in.o: In function `imx_otg_ulpi_create':
arch/arm/mach-imx/ulpi.c:117: undefined reference to `otg_ulpi_create'
arch/arm/mach-omap2/built-in.o: In function `usbhs_init_phys':
arch/arm/mach-omap2/usb-host.c:652: undefined reference to `usb_bind_phy'
arch/arm/mach-omap2/built-in.o: In function `omap_2430sdp_init':
arch/arm/mach-omap2/board-2430sdp.c:236: undefined reference to `usb_bind_phy'
arch/arm/mach-omap2/built-in.o: In function `omap3_beagle_init':
arch/arm/mach-omap2/board-omap3beagle.c:554: undefined reference to `usb_bind_phy'
arch/arm/mach-omap2/built-in.o: In function `devkit8000_init':
arch/arm/mach-omap2/board-devkit8000.c:596: undefined reference to `usb_bind_phy'
arch/arm/mach-omap2/built-in.o: In function `omap_ldp_init':
arch/arm/mach-omap2/board-ldp.c:379: undefined reference to `usb_bind_phy'
drivers/built-in.o: In function `ab8500_charger_probe':
drivers/power/ab8500_charger.c:3629: undefined reference to `usb_get_phy'
drivers/power/ab8500_charger.c:3706: undefined reference to `usb_put_phy'
drivers/built-in.o: In function `ab8500_charger_remove':
drivers/power/ab8500_charger.c:3411: undefined reference to `usb_put_phy'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>