rndis_wlan: get max_num_pmkids from device

Extend rndis_wlan_get_caps() to get 802.11 capabilities and maximum
supported number of PMKIDs by device.

v2: fixed to use new netdev_dbg/warn/etc instead of old devdbg/warn/etc

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>