cmd: ethsw: Fix out-of-bounds error

The for loop in cmd_keywords_opt_check() seems to use wrong array to
set boundary, reported by Coverity analysis.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Joe Hershberger <joe.hershberger@ni.com>
CC: Tom Rini <trini@konsulko.com>
CC: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reported-by: Coverity (CID: 163251)
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>