wifi: rtl8xxxu: fix mixed declarations in rtl8xxxu_set_aifs()

Moving struct ieee80211_sta *sta variable definition to the front
of the code to fix the ISO C90 forbids mixed declarations and code
warning.

Fixes: 43532c05

 ("wifi: rtl8xxxu: support multiple interfaces in set_aifs()")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/TYAP286MB03157A408E0D69F2F6FBD88ABC552@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM