broadcom: tg3: fix use of SPEED_UNKNOWN ethtool constant

tg3 driver uses u16 to store SPEED_UKNOWN ethtool constant,
which is defined as -1, resulting in value truncation and
thus incorrect test results against SPEED_UNKNOWN.

For example, the following test will print "False":

	u16 speed = SPEED_UNKNOWN;

	if (speed == SPEED_UNKNOWN)
	    printf("True");
	else
	    printf("False");

Change storage of speed to use u32 to avoid this issue.

Signed-off-by: Michael Zhivich <mzhivich@akamai.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>