ramips: fix reboot with W25Q256 with 4-address-mode enabled

ramips: fix reboot with W25Q256 with 4-address-mode enabled

Some board vendors actually changed the loader to expect the chip
to come up in 4-address-mode and flipped the ADP bit in the flash
chip's configuration register which makes it come up in 4-address-mode.
Hence it doesn't make sense to avoid switching to 4-address-mode on
those boards but the opposite as otherwise reboot hangs eg. on the
WrtNode2 boards. Fix this by checking the ADP register and only using
SPI_NOR_4B_READ_OP on chips which have ADP==0 (come up in 3-byte mode).

See also datasheet section 7.1.11 Power Up Address Mode (ADP)

Fixes: 22d982ea0 ("ramips: add support for switching between 3-byte and 4-byte addressing on w25q256 flash")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(backported from 8796680277f4b231386fbc6e8a1ccae91d9fcbe8)