char/tpm/tpm_i2c_stm_st33: Don't use memcpy for one byte assignment

We don't need to call memcpy for one byte, but assign it directly.
And to make the offset clearer we use the array syntax on the subsequent
call to memset to make the relationship clearer.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Kent Yoder <key@linux.vnet.ibm.com>