Since for MMIO driver using FIFO registers, also known as tpm_tis, the
default (and tbh recommended) behaviour is now the polling mode, the
"tristate" workaround is no longer for benefit.

If someone wants to explicitly enable IRQs for a TPM chip that should be
without question allowed. It could very well be a piece hardware in the
existing deny list because of e.g. firmware update or something similar.

While at it, document the module parameter, as this was not done in 2006
when it first appeared in the mainline.

Link: https://lore.kernel.org/linux-integrity/20201015214430.17937-1-jsnitsel@redhat.com/
Link: https://lore.kernel.org/all/1145393776.4829.19.camel@localhost.localdomain/Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>