common: env_flags: fix loop condition when using env_flags_varaccess_mask

We should use ARRAY_SIZE, but not sizeof. The size of
env_flags_varaccess_mask is 16bytes, but we only need 4 loops.
If using 16 as the end condition, we may access memory that
not belong to array env_flags_varaccess_mask.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: York Sun <yorksun@freescale.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>