rtlwifi: cleanup the code that check whether TX ring is available

Remove the duplicate checking of TX ring's available number, and remove
the variable to store available number that can be calculated by
read/write pointers.

Signed-off-by: Steven Ting <steventing@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>