rtwn: only set INIRATE if we're not doing firmware rate control.

This register (array) controls the initial rate to use for each
MACID. There's no need to set it if firmware rate control is enabled -
it'll actually be under firmware control (and we can read it back to
see what choices the firmware is making.)

Locally tested:

• RTL8188EU, STA
• RTL8192CU, STA
• RTL8192EU, STA

Differential Revision: https://reviews.freebsd.org/D48094
Reviewed by: bz