iwm: add sku_cap_mimo_disable; populate sku_cap_11n_enable

This is a flag to be used to disable >1 antenna support, even if
the radio supports it.

Also populate sku_cap_11n_enable, which indicates HT is available.

It's a no-op right now; it's one of the requirements for properly
configuring HT support and transmit/receive antenna configurations.

Obtained from: OpenBSD (at a6e25c727a2214219715b51ba6af2221d7e0d674, 22-Oct-2021)
Differential Revision: https://reviews.freebsd.org/D48210
Reviewed by: bz