tpm2_i2c_nuvoton: add irq validity check

In 570a3609 IRQ path is incorrectly always exercised while it should be
exercised only when there is an IRQ number allocated. This commit
reverts the old behavior.

[jarkko.sakkinen@linux.intel.com: updated description]

Fixes: 570a3609

 ("tpm: drop 'irq' from struct tpm_vendor_specific")
Signed-off-by: Andrew Zamansky <andrew.zamansky@nuvoton.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>