fpga: altera: Make altera_validate return normal values

fpga: altera: Make altera_validate return normal values

Make the function return either 0 or -EINVAL, that is, normal
expected error codes and success codes instead of true/false
nonsense.

Signed-off-by: Marek Vasut <[email protected]>
Cc: Chin Liang See <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Albert Aribaud <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Pavel Machek <[email protected]>
Acked-by: Pavel Machek <[email protected]>