tpm: Fix the type of the return value in calc_tpm2_event_size()

calc_tpm2_event_size() has an invalid signature because
it returns a 'size_t' where as its signature says that
it returns 'int'.

Cc: <stable@vger.kernel.org>
Fixes: 4d23cc32 ("tpm: add securityfs support for TPM 2.0 firmware event log")
Suggested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>