To make hardware efficient to determine if RX ring is full, introduce new
design that checks if reading and writing indices are equal. Comparing
to old design, initial indices of both reading and writing indices are 0
that means empty, and hardware checks full by "writing index + 1 ==
reading index". The "+1" has extra cost for hardware, so new design is
to avoid this.

Take ring size is 256 as an example, the initial reading and writing
indices are 255 and 0 respectively; the initial values mean empty. If two
indices are the same, for example 5 and 5, it means ring is full.

   wp       rp       used_cnt        state
   255      0        0               initial (ring is empty)
   255      1        1               receive 1st packet
   255      2        2               receive 2nd packet
   0        2        1               driver read 1st packet
   1        2        0               driver read 2nd packet (ring is empty)
            :
   5        5        255             ring is full

Note: 'rp' is hardware writing index
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231026120049.9187-4-pkshih@realtek.com