net: stmmac: add helper to set receive tail pointer

net: stmmac: add helper to set receive tail pointer

Setting the queue receive tail pointer follows a common pattern:
calculate the DMA address, and then call stmmac_set_rx_tail_ptr().
The only difference between all the call sites is the index used.

Factor this out into a static function, and add a comment about why
it only uses the normal descriptor size.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w1LWE-0000000DGSF-2Hbs@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>