brcmfmac: not set mbss in vif if firmware does not support MBSS

With RSDB mode, FMAC and firmware are able to create 2 or more AP,
so we should not set mbss in vif structure if firmware does not
support MBSS feature.

Signed-off-by: Wright Feng <wright.feng@cypress.com>
Signed-off-by: Soeren Moch <smoch@web.de>
Reviewed-by: Chi-Hsien Lin <chi-hsien.lin@cypress.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>