net: fec: correct rx_bytes statistic for the case SHIFT16 is set

net: fec: correct rx_bytes statistic for the case SHIFT16 is set

Two additional bytes in front of each frame received into the RX FIFO if
SHIFT16 is set, so we need to subtract the extra two bytes from pkt_len
to correct the statistic of rx_bytes.

Fixes: 3ac72b7b63d5 ("net: fec: align IP header in hardware")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20251106021421.2096585-1-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>