examples/l2fwd-crypto: fix supported key size check

examples/l2fwd-crypto: fix supported key size check

When initializing crypto devices within the app,
the provided key sizes are checked against the supported
sizes from the crypto device capabilities.

When the supported sizes are not a range, but a single value,
the check may become an infinite loop (when size is not supported).

Fixes: a061e50a0d97 ("examples/l2fwd-crypto: fix ambiguous input key size")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Tested-by: Min Cao <min.cao@intel.com>