boot: fix bootdev_list()

boot: fix bootdev_list()

uclass_get_device_by_name() is meant to return 0 or a negative error code.
simple_itoa() cannot handle negative numbers.

This leads to output like:

    => bootdev list -p

    Seq  Probed  Status  Uclass    Name
    ---  ------  ------  --------  ------------------
      c   [   ]  18446744073709551614  spi_flash spi.bin@0.bootdev

Convert the status to a positive number. Now we get

    Seq  Probed  Status  Uclass    Name
    ---  ------  ------  --------  ------------------
      c   [   ]       2  spi_flash spi.bin@0.bootdev

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>