Function _rtl8821ae_mrate_idx_to_arfr_id is functionally identical to
the generic version rtl_mrate_idx_to_arfr_id, so remove
_rtl8821ae_mrate_idx_to_arfr_id and use the generic one instead.

This also fixes a missing break statement found by CoverityScan in
_rtl8821ae_mrate_idx_to_arfr_id, namely: CID#1167237 ("Missing break
in switch")

Thanks to Joe Perches for spotting this when I submitted an earlier patch.

Fixes: 3c05bedb ("Staging: rtl8812ae: Add Realtek 8821 PCI WIFI driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
ACKed-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>