brcmfmac: fix NULL pointer access in brcmf_create_iovar()

The function brcmf_fil_bsscfg_data_get() calls brcmf_create_iovar()
with data pointer set to NULL, which caused a NULL pointer access.
As it should be possible to provide data in message towards the
firmware, it should just pass the data buffer instead.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>