ssb: drop BROKEN from SSB_SFLASH

With recent patches ssb can fetch info about serial flash and register
it as a platform device. No more reasons to mark it BROKEN.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>