rtwn: create a new HAL routine for enabling STA mode beacon processing

For some NICs (notably the rtl8192cu that I'm working on) the
firmware rate adaptation requires beacon processing to be enabled.

Instead of making assumptions in the if_rtwn beacon routines (and
honestly all of that should be in the HAL too), create a HAL method
for enabling/disabling beacon processing specifically in STA mode.

Since this isn't necessarily required for all NICs (notably the RTL8188E
NICs, where some will do firmware rate control and some will require
driver rate control), only enable it for the RTL8192CU and RT8192EU.

The RTL8188E and RTL8812/RTL8821 just have no-op routines for now.

Locally tested:

• RTL8192CU, STA mode

Differential Revision: https://reviews.freebsd.org/D48066
Reviewed by: bz