vhost/crypto: fix build with gcc 4.7.2

vhost/crypto: fix build with gcc 4.7.2

Build error has been reported by Intel build system:
SUSE12SP3_64 / Linux 3.7.10-1 / GCC 4.7.2
lib/librte_vhost/vhost_crypto.c: In function ‘rte_vhost_crypto_set_zero_copy’:
lib/librte_vhost/vhost_crypto.c:1192:2: error:
comparison of unsigned expression < 0 is always false

As enums can be either signed or unsigned, this patch removes
the negative check and cast to unsigned the upper limit check.

Fixes: 939066d96563 ("vhost/crypto: add public function implementation")

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>