brcmfmac: Correct mcs index report

There is a mismatch between the mcs index(0-7) reported to cfg80211
and the actual mcs index(0-15) in use. This patch resolves the mismatch
by setting mcs info with the number of chains read from FW.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Daniel Kim <dekim@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>