arm: stm32f4: fix a bug when only first sector gets erased

flash_lock call is inside a for loop, so after the first iteration flash
is locked and no more sectors can be erased.
Move flash_lock out of the loop.

Signed-off-by: Vadzim Dambrouski <pftbest@gmail.com>

arch/arm/cpu/armv7m/stm32f4/flash.c

@@ -114,9 +114,9 @@ int flash_erase(flash_info_t *info, int first, int last)
 			;
 
 		clrbits_le32(&STM32_FLASH->cr, STM32_FLASH_CR_SER);
-		stm32f4_flash_lock(1);
 	}
 
+	stm32f4_flash_lock(1);
 	return 0;
 }