rtwn: allow firmware rate control to be enabled for rtl8192cu

Although there's no RTS/CTS rate control available yet, the support
is enough to enable firmware rate control for experimenting.

This won't be enabled by default - users will need to set a tunable
before loading the driver (eg kenv dev.rtwn.0.ratectl=2) but it is
enough for experimentation, feedback and continued work.

Locally tested:

• RTL8192CU, STA mode

Differential Revision: https://reviews.freebsd.org/D48143
Reviewed by: bz, emaste