net80211: allow to override SWCRYPT/SWMIC bits in the driver

Add IEEE80211_KEY_SWCRYPT / IEEE80211_KEY_SWMIC bits to the
IEEE80211_KEY_DEVICE mask - as a result, those bits will be preserved
during group key handshake.

A driver can override them in iv_key_alloc() for some keys in case
when hardware crypto support is not possible. As an example:

• multi-vap without multicast key search support;
• IBSS RSN for devices w/ fixed storage for group keys;

Tested with RTL8188EU (AP, sw crypto) and
RTL8821AU (STA, sw crypto for group keys + hw crypto for pairwise keys)

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D7901