staging:iio:dac:ad5504: Check if IRQ was requested before freeing it

IRQ support is optional, so we have to check whether it was actually requested
before we attempt to free it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>