rtlwifi: rtl8192se: remove duplicated legacy_httxpowerdiff

The legacy_httxpowerdiff in rtl8192se is pretty much the same as
the legacy_ht_txpowerdiff for other chips. Use the same name to
keep the consistency.

Signed-off-by: Chris Chiu <chiu@endlessos.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201006035928.5566-1-chiu@endlessm.com