r8169,sis190: remove unnecessary length check

The ethtool core will lower the requested length to the one returned by
get_regs_len, therefore no additional check is needed in the get_regs
function.

Reported-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Peter Wu <lekensteyn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>