mtd: cfi: respect reg address length

flash_get_size() will get the flash size from the device itself and go
through all erase regions to read protection status. However, the device
mappable region (eg: devicetree reg property) might be lower than the
device full size which means that the above cycle will result in a data
bus exception. This change fixes it by reading the 'addr_size' during
probe() and also use that as one possible upper limit.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>