net: phy: mscc: fix signedness bug in vsc85xx_downshift_get

Currently, the error handling for the call to function
phy_read_paged() doesn't work because *reg_val* is of
type u16 (16 bits, unsigned), which makes it impossible
for it to hold a value less than 0.

Fix this by changing the type of variable *reg_val* to int.

Addresses-Coverity-ID: 1473970 ("Unsigned compared against 0")
Fixes: 6a0bfbbe

 ("net: phy: mscc: migrate to phy_select/restore_page functions")
Reviewed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>