mtd: nand: split ONFI detection logic to its own function

In order to reduce the indentation and improve the readability of nand_get_
flash_type, split the ONFI detection logic to its own function. The detection
logic inside nand_flash_detect_onfi is also rewritten to allow for less
indentation.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>