mwifiex: Add support for NL80211_ATTR_MAX_AP_ASSOC_STA

SD8997 firmware sends TLV_TYPE_MAX_CONN with struct hw_spec_max_conn to
inform kernel about maximum number of p2p connections and stations in AP
mode.

During initialization of SD8997 wifi chip kernel prints warning:

  mwifiex_sdio mmc0:0001:1: Unknown GET_HW_SPEC TLV type: 0x217

This patch adds support for parsing TLV_TYPE_MAX_CONN (0x217) and sets
appropriate cfg80211 member 'max_ap_assoc_sta' from retrieved structure.

It allows userspace to retrieve NL80211_ATTR_MAX_AP_ASSOC_STA attribute.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200521123559.29028-1-pali@kernel.org