[][kernel][common][eth][Fix issues detected by Coverity scan]

[Description]
Fix issues detected by Coverity scan.
mtk_rx_alloc: OVERFLOW_BEFORE_WIDEN

[Release-log]
N/A


Change-Id: I887fd79b7cc8fd7c660dbc95ecb2ec92a1e147b7
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8891297
1 file changed