net: phy: at803x: Avoid comparison is always false warning

By placing the GENMASK value into an unsigned int and then passing it
to PREF_FIELD, the type is reduces down from ULL. Given the reduced
size of the type, the range checks in PREP_FAIL() are always true, and
-Wtype-limits then gives a warning.

By skipping the intermediate variable, the warning can be avoided.

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>