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: 47b8edbf ("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>