common: env_flags: fix loop condition when using env_flags_varaccess_mask

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 <[email protected]>
Cc: Joe Hershberger <[email protected]>
Cc: York Sun <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Acked-by: Joe Hershberger <[email protected]>