net: airoha: simplify rx/free packet logic a bit

The commit 997786bbf473 ("drivers/net/airoha_eth: fix stalling in package
receiving") can be improved. Instead of returning previous descriptor
it's possible:
 * do nothing in even descriptor case
 * return 2 descriptor to the queue (current and previous) in the odd
   descriptor case.

This patch:
 * implements above approach
 * remove logic not required within new approach
 * adds note that PKTBUFSRX must be even and larger than 7
   for reliable driver operations

Signed-off-by: Mikhail Kshevetskiy <[email protected]>

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