Remove the function rtl92ee_get_available_desc() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: NRickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Acked-by: NLarry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>