tpm: xen-tpmfront: Use struct_size() helper

tpm: xen-tpmfront: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worse scenario, could lead to heap overflows.

Also, address the following sparse warning:
drivers/char/tpm/xen-tpmfront.c:131:16: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/160
Link: https://github.com/KSPP/linux/issues/174
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>