mtd: spi-nor: Fix lock/unlock on Micron with more than 896 sectors.

This fix sector pass 56MB to be impossible to unlock on large chip.
Should only affect Micron chip although similar fix might be needed
for chips from other vendors that make use of SR_BP3 bit to lock.

Tested on Micron N25Q512A chip, should apply to all Micron chips that
uses a fourth (SR_BP3) protection bit.

Signed-off-by: William Bourque <wfb@xiphos.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>