ssb: fix NULL ptr deref when pcihost_wrapper is used

Ethernet driver b44 does register ssb by it's pcihost_wrapper
and doesn't set ssb_chipcommon. A check on this value
introduced with commit d53cdbb9
and ea2db495 triggers:

BUG: unable to handle kernel NULL pointer dereference at 00000010
IP: [<c1266c36>] ssb_is_sprom_available+0x16/0x30

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>