rtw88: rearrange if..else statements for rx rate indexes

Driver just memset() rx_status to 0 before assigning rate indexes.
And driver could never hit the 'else' because the driver checks
if 'pkt_stat->rate >= DESC_RATEMCS0', so the 'else' statement can
be removed.

Also rearrange the if..else statements because DESC_RATEMCS0 is
actually larger than DESC_RATE1M ~ DESC_RATE54M, move the check
of 'pkt_stat->rate >= DESC_RATEMCS0' to the last to keep an
increasing order.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>