USB: fix ssb_ohci_probe() build bug

fix ssb_ohci_probe() build bug:

 drivers/built-in.o: In function `ssb_ohci_probe':
 ohci-hcd.c:(.text+0xbff39): undefined reference to `ssb_device_enable'
 ohci-hcd.c:(.text+0xbff6f): undefined reference to `ssb_admatch_base'
 ohci-hcd.c:(.text+0xbff8b): undefined reference to `ssb_admatch_size'
 ohci-hcd.c:(.text+0xbffe5): undefined reference to `ssb_device_disable'
 [...]

the reason was that this Kconfig combination was allowed:

 CONFIG_SSB=m
 CONFIG_USB_OHCI_HCD=y
 CONFIG_USB_OHCI_HCD_SSB=y

the fix is to require a modular USB_OHCI_HCD build when SSB is modular.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>