SSB: fix Kconfig dependencies

SSB: fix Kconfig dependencies

The commit 21400f252a97 ("MIPS: BCM47XX: Make ssb init NVRAM instead of
bcm47xx polling it") introduces a dependency to SSB_SFLASH but did not
add it to the Kconfig.

drivers/ssb/driver_mipscore.c:216:36: error: 'struct ssb_mipscore' has no
member named 'sflash'
  struct ssb_sflash *sflash = &mcore->sflash;
                                    ^
drivers/ssb/driver_mipscore.c:249:12: error: dereferencing pointer to
incomplete type
  if (sflash->present) {
            ^

Signed-off-by: Adrien Schildknecht <adrien+dev@schischi.me>
Cc: m@bues.ch
Cc: zajec5@gmail.com
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9598/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>