net: phy: bcm54140: fix less than zero comparison on an unsigned

Currently the unsigned variable tmp is being checked for an negative
error return from the call to bcm_phy_read_rdb and this can never
be true since tmp is unsigned.  Fix this by making tmp a plain int.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: 4406d36d ("net: phy: bcm54140: add hwmon support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>