ethtool: remove unnecessary check in ethtool_get_regs()

We recently changed this function in commit f9fc54d3 ("ethtool:
check the return value of get_regs_len") such that if "reglen" is zero
we return directly.  That means we can remove this condition as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>