net: pcs: xpcs: Fix a less than zero u16 comparison error

Currently the check for the u16 variable val being less than zero is
always false because val is unsigned. Fix this by using the int
variable for the assignment and less than zero check.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: f7380bba ("net: pcs: xpcs: add support for NXP SJA1110")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>