trusted-keys: check hex2bin result

For each hex2bin call in trusted keys, check that the ascii hex string is
valid.  On failure, return -EINVAL.

Changelog v1:
- hex2bin now returns an int

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>