The "nb_firewall" variable is the number of elements in the firewall[]
array, which is allocated in stm32_firewall_populate_bus().  So change
this > comparison to >= to prevent an out of bound access.

Fixes: 5c9668cf ("firewall: introduce stm32_firewall framework")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>