ethtool: propagate get_coalesce return value

get_coalesce returns 0 or ERRNO, but the return value isn't checked.
The returned coalesce data may be invalid if an ERRNO is set,
therefore better check and propagate the return value.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>