fpga: socfpga: Fix check of return value of devm_request_irq

The return value should be checked for non-zero, instead
of checking it being IS_ERR_VALUE().

Acked-by: Alan Tull <atull@opensource.altera.com>
Reviewed-by: Josh Cartwright <joshc@eso.teric.us>
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>