tg3: fix bug caused by uninitialized variable

The reported bug is caused by using mii_eee_cap1_mod_linkmode_t()
with an uninitialized bitmap. Fix this by zero-initializing the
struct containing the bitmap.

Fixes: 9bc79134 ("tg3: convert EEE handling to use linkmode bitmaps")
Reported-by: Srikanth Aithal <sraithal@amd.com>
Tested-by: Srikanth Aithal <sraithal@amd.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>