crypto/dpaa2_sec: fix build with GCC 7

crypto/dpaa2_sec: fix build with GCC 7

Seen with GCC 7.2.0, a switch fall through is detected and
cannot be fixed with a fall-through comment or attribute:

drivers/crypto/dpaa2_sec/hw/rta/operation_cmd.h:89:6: error:
this statement may fall through [-Werror=implicit-fallthrough=]
   if (rta_sec_era < RTA_SEC_ERA_2)
      ^

The check is disabled in dpaa2_sec Makefile but not in dpaa_sec Makefile
which uses source code shared by dpaa2_sec.

The workaround is to disable the check at the beginning of the file.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>