staging:iio:ad799x fix error_free_irq which was freeing an irq that may not have been requested

commit 38408d056188be29a6c4e17f3703c796551bb330 upstream.

Only free an IRQ in error_free_irq, if it has been requested previously.

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