ethtool: avoid signed-unsigned comparison in ethtool_validate_speed()

When building C++ userspace code that includes ethtool.h
with "-Werror -Wall", g++ complains about signed-unsigned comparison in
ethtool_validate_speed() due to definition of SPEED_UNKNOWN as -1.

Explicitly cast SPEED_UNKNOWN to __u32 to match type of
ethtool_validate_speed() argument.

Signed-off-by: Michael Zhivich <mzhivich@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>