platform: cznic: turris-omnia-mcu: Fix error check in omnia_mcu_register_trng()

The gpiod_to_irq() function never returns zero.  It returns negative
error codes or a positive IRQ number.  Update the checking to check
for negatives.

Fixes: 41bb142a ("platform: cznic: turris-omnia-mcu: Add support for MCU provided TRNG")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>