net: phy: dp83869: fix unsigned comparisons against less than zero values

Currently the comparisons of u16 integers value and sopass_val with
less than zero for error checking is always false because the values
are unsigned. Fix this by making these variables int.  This does not
affect the shift and mask operations performed on these variables

Addresses-Coverity: ("Unsigned compared against zero")
Fixes: 49fc2301 ("net: phy: dp83869: support Wake on LAN")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>