crypto: caam - fix operator precedence in shared descriptor allocation

setkey allocates 16 bytes (CAAM_CMD_SZ *
DESC_AEAD_SHARED_TEXT_LEN) shy of what is needed to
store the shared descriptor, resulting in memory
corruption.  Fix this.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>