tpm_tis: Do *not* flush uninitialized work

tpm_tis_core_init() may fail before tpm_tis_probe_irq_single() is
called, in which case tpm_tis_remove() unconditionally calling
flush_work() is triggering a warning for .func still being NULL.

Cc: stable@vger.kernel.org # v6.5+
Fixes: 481c2d14 ("tpm,tpm_tis: Disable interrupts after 1000 unhandled IRQs")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>