wifi: rtl8xxxu: tighten bounds checking in rtl8xxxu_read_efuse()

There some bounds checking to ensure that "map_addr" is not out of
bounds before the start of the loop.  But the checking needs to be
done as we iterate through the loop because "map_addr" gets larger as
we iterate.

Fixes: 26f1fad2 ("New driver: rtl8xxxu (mac80211)")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jes Sorensen <Jes.Sorensen@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/Yv8eGLdBslLAk3Ct@kili