tpm/tpm_ppi: Do not compare strcmp(a,b) == -1

Depending on the implementation strcmp might return the difference between
two strings not only -1,0,1 consequently
 if (strcmp (a,b) == -1)
might lead to taking the wrong branch

-> compare with < 0  instead,
which in any case is more canonical.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>