bus: stm32_firewall: fix off by one in stm32_firewall_get_firewall()

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: 5c9668cfc6d7 ("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>

firewall: introduce stm32_firewall framework

Introduce a STM32 firewall framework that offers to firewall consumers
different firewall services such as the ability to check their access
rights against their firewall controller(s).

The STM32 firewall framework offers a generic API for STM32 firewall
controllers that is defined in their drivers to best fit the
specificity of each firewall.

There are various types of firewalls:
-Peripheral firewalls that filter accesses to peripherals
-Memory firewalls that filter accesses to memories or memory regions
-No type for undefined type of firewall

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>