drivers: entropy: fail mcux CAAM init on error

Commit d556a0c8
("drivers: entropy: Add entropy driver for MCUX CAAM")
added a shim entropy driver whose initialization function
always returns 0, even when the underlying HAL API fails.

This is wrong; if the device initialization function fails, it must
return nonzero by contract. Papering this over with an assert is not
enough. Fix it by returning -ENODEV on error.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>