brcmfmac: reduce maximum station interface from 2 to 1 in RSDB mode

The firmware state machines are not fully suitable for concurrent
station interface support, it may hit unexpected error if we have 2
different SSIDs and the roaming scenarios concurrently.
To avoid the bad user-experience if this is not fully validated, we
dis-allow user to create two concurrent station interfaces.

Signed-off-by: Wright Feng <wright.feng@cypress.com>
Signed-off-by: Chi-hsien Lin <chi-hsien.lin@cypress.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200604071835.3842-5-wright.feng@cypress.com