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>

include/uapi/linux/ethtool.h

@@ -1591,7 +1591,7 @@ enum ethtool_link_mode_bit_indices {
 
 static inline int ethtool_validate_speed(__u32 speed)
 {
-	return speed <= INT_MAX || speed == SPEED_UNKNOWN;
+	return speed <= INT_MAX || speed == (__u32)SPEED_UNKNOWN;
 }
 
 /* Duplex, half or full. */