firewall: remove misplaced semicolon from stm32_firewall_get_firewall

Remove misplaced colon in stm32_firewall_get_firewall()
which results in a syntax error when the code is compiled
without CONFIG_STM32_FIREWALL.

Fixes: 5c9668cf ("firewall: introduce stm32_firewall framework")
Signed-off-by: guanjing <guanjing@cmss.chinamobile.com>
Reviewed-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>