mtd: spi-nor: fix wrong "fully unlocked" test

In stm_unlock(), the test to determine whether we've fully unlocked the
flash checks for the lock length to be equal to the flash size. That is
a typo/think-o -- the condition actually means the flash is completely
*locked.* We should be using the inverse condition -- that the lock
length is 0 (i.e., no protection).

The result of this bug is that we never actually turn off the Status
Register Write Disable bit, even if the flash is completely unlocked.
Now we can.

Fixes: 47b8edbf0d43 ("mtd: spi-nor: disallow further writes to SR if WP# is low")
Reported-by: Giorgio <giorgio.nicole@arcor.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>

drivers/mtd/spi-nor/spi-nor.c

@@ -661,7 +661,7 @@ static int stm_unlock(struct spi_nor *nor, loff_t ofs, uint64_t len)
 	status_new = (status_old & ~mask & ~SR_TB) | val;
 
 	/* Don't protect status register if we're fully unlocked */
-	if (lock_len == mtd->size)
+	if (lock_len == 0)
 		status_new &= ~SR_SRWD;
 
 	if (!use_top)