MIPS: BMC47xx: fix kconfig dependency bug for BCM47XX_SSB

MIPS: BMC47xx: fix kconfig dependency bug for BCM47XX_SSB

When BCM47XX_SSB is enabled and SSB_PCIHOST is disabled, it results in the
following Kbuild warning:

WARNING: unmet direct dependencies detected for SSB_B43_PCI_BRIDGE
  Depends on [n]: SSB [=y] && SSB_PCIHOST [=n]
  Selected by [y]:
  - BCM47XX_SSB [=y] && BCM47XX [=y] && PCI [=y]

The reason is that BCM47XX_SSB selects SSB_B43_PCI_BRIDGE without
depending on or selecting SSB_PCIHOST while SSB_B43_PCI_BRIDGE depends on
SSB_PCIHOST. This can also fail building the kernel as demonstrated in a
bug report.

Honor the kconfig dependency to remove unmet direct dependency warnings
and avoid any potential build failures.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=210051
Signed-off-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>