DEF_HELPER_FLAGS_5 was added some time ago without adjusting
MAX_OPC_PARAM_IARGS.

Fixing the definition becomes more important as QEMU is using
an increasing number of helper functions called with 5 arguments.

Add also a comment to avoid future problems when DEF_HELPER_FLAGS_6
will be added.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>