stm32f4: Fix SBF clear bit
The STM32F412 and STM32F446 reference manuals seem
to indicate that the SBF clear bit is actually bit 3.
BRANCH=hatch
TEST=make buildall -j
#define STM32_PWR_RESET_CAUSE STM32_PWR_CSR
#define RESET_CAUSE_SBF BIT(1)
#define STM32_PWR_RESET_CAUSE_CLR STM32_PWR_CR
-#define RESET_CAUSE_SBF_CLR BIT(2)
+#define RESET_CAUSE_SBF_CLR BIT(3)
/* --- Watchdogs --- */
