net: phy: mscc: shorten `x != 0` condition to `x`

`if (x != 0)` is basically a more verbose version of `if (x)` so let's
use the latter so it's consistent throughout the whole driver.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>