mtd: st_spi_fsm: Fix [-Wsign-compare] build warning

drivers/mtd/devices/st_spi_fsm.c:1647:17:
  warning: comparison between signed and unsigned integer expressions

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/devices/st_spi_fsm.c

@@ -1586,11 +1586,11 @@ static int stfsm_write(struct stfsm *fsm, const uint8_t *buf,
 	uint32_t size_lb;
 	uint32_t size_mop;
 	uint32_t tmp[4];
+	uint32_t i;
 	uint32_t page_buf[FLASH_PAGESIZE_32];
 	uint8_t *t = (uint8_t *)&tmp;
 	const uint8_t *p;
 	int ret;
-	int i;
 
 	dev_dbg(fsm->dev, "writing %d bytes to 0x%08x\n", size, offset);