phy: mvebu-cp110-comphy: fix mux error check

The mux value is retrieved from the mvebu_comphy_get_mux() function
which returns an int. In mvebu_comphy_power_on() this int is stored to a
u32 and a check is made to ensure it's not negative. Which is wrong.
This fixes it.

Fixes: d0438bd6 ("phy: add the mvebu cp110 comphy driver")
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>