tpm: fix invalid return value in pubek_show()

Return zero when tpm_buf_init() fails as we do for other functions in
tpm-sysfs.c.

Fixes: da379f3c

 ("tpm: migrate pubek_show to struct tpm_buf")
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Tested-by: Alexander Steffen <Alexander.Steffen@infineon.com>