net: stmmac: selftests: Use kcalloc() instead of kzalloc()

Use 2-factor multiplication argument form kcalloc() instead
of kzalloc().

Link: https://github.com/KSPP/linux/issues/162


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20211006180944.GA913477@embeddedor


Signed-off-by: Jakub Kicinski <kuba@kernel.org>