ssb: make SSB_PCICORE_HOSTMODE depend on SSB = y

SSB_PCICORE_HOSTMODE protects MIPS specific code that calls not exported
symbols pcibios_enable_device and register_pci_controller. This code is
supposed to be compiled only with ssb builtin.

This fixes:
ERROR: "pcibios_enable_device" [drivers/ssb/ssb.ko] undefined!
ERROR: "register_pci_controller" [drivers/ssb/ssb.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:92: __modpost] Error 1

Reported-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>