mtd: nand: raw: atmel: Add error handling when rb-gpios missing

mtd: nand: raw: atmel: Add error handling when rb-gpios missing

Adapt behaviour to Linux kernel driver.

The return value of gpio_request_by_name_nodev() was not checked before,
and thus in case 'rb-gpios' was missing in DT, rb.type was set to
ATMEL_NAND_GPIO_RB nevertheless, leading to output like this for
example (on sam9x60-curiosity with the line removed from dts):

    NAND:  Could not find valid ONFI parameter page; aborting
    device found, Manufacturer ID: 0xc2, Chip ID: 0xdc
    Macronix NAND 512MiB 3,3V 8-bit
    512 MiB, SLC, erase size: 256 KiB, page size: 4096, OOB size: 64
    atmel-nand-controller nand-controller: NAND scan failed: -22
    Failed to probe nand driver (err = -22)
    Failed to initialize NAND controller. (error -22)
    0 MiB

Note: not having that gpio assigned in dts is possible, the driver does
not override nand_chip->dev_ready() then and a generic solution is used.

Fixes: 6a8dfd57220d ("nand: atmel: Add DM based NAND driver")
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Eugen Hristev <eugen.hristev@collabora.com>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>