[][kernel][common][eth][Fix Coverity scan warning]

[Description]
Fix Coverity scan warning.

Without this patch, the Coverity might detect "Unsigned compared against
0 issue" warning in the mtk_add_mux().

[Release-log]


Change-Id: I7a7c3c7707063a7a0fd1d6b2c9564d0f0b49be45
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8061023
1 file changed