rtl8xxxu: fill up txrate info for gen1 chips

RTL8188CUS/RTL8192CU(gen1) don't support rate adaptive report hence
no real txrate info can be retrieved. The vendor driver reports the
highest rate in HT capabilities from the IEs to avoid empty txrate.
This commit initiates the txrate information with the highest supported
rate negotiated with AP. The gen2 chip keeps update the txrate from
the rate adaptive reports, and gen1 chips at least have non-NULL txrate
after associated.

Signed-off-by: Chris Chiu <chris.chiu@canonical.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220325035735.4745-3-chris.chiu@canonical.com