b43: make B43_PCMCIA and B43_SDIO depend on B43_SSB

These are extra configs specific to the SSB. Lack of this dependency and
CONFIG_B43=y
CONFIG_B43_BUSES_BCMA=y
CONFIG_SSB=m
would result in:
> drivers/built-in.o: In function `b43_sdio_remove':
> sdio.c:(.text+0x14657f): undefined reference to `ssb_bus_unregister'
> drivers/built-in.o: In function `b43_sdio_probe':
> sdio.c:(.text+0x14672f): undefined reference to `ssb_bus_sdiobus_register'

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>