mtd: spi-nor: ignore errors in spi_nor_unlock_all()

mtd: spi-nor: ignore errors in spi_nor_unlock_all()

Just try to unlock the whole SPI-NOR flash array. Don't abort the
probing in case of an error. Justifications:
 (1) For some boards, this just works because
     spi_nor_write_16bit_sr_and_check() is broken and just checks the
     second half of the 16bit. Once that will be fixed, SPI probe will
     fail for boards which has hardware-write protected SPI-NOR flashes.
 (2) Until now, hardware write-protection was the only viable solution
     to use the block protection bits. This is because this very
     function spi_nor_unlock_all() will be called unconditionally on
     every linux boot. Therefore, this bits only makes sense in
     combination with the hardware write-protection. If we would fail
     the SPI probe on an error in spi_nor_unlock_all() we'd break
     virtually all users of the block protection bits.
 (3) We should try hard to keep the MTD working even if the flash might
     not be writable/erasable.

Fixes: 3e0930f109e7 ("mtd: spi-nor: Rework the disabling of block write protection")
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201203162959.29589-3-michael@walle.cc