spl: blk: partition numbers are hexadecimal

Loading u-boot.itb from device 0x00, partition 0x0f fails with:

    Trying to boot from NVME

    Device 0: Vendor: 0x4x Rev: 8.0.50   Prod: nvme-1
                Type: Hard Disk
                Capacity: 3814.6 MB = 3.7 GB (7812500 x 512)
    ** Invalid partition 21 **
    Couldn't find partition nvme 0:15

Like the command line interface fs_det_blk_dev() expects that the device
number and the partition number are hexadecimal.

Fixes: 8ce6a2e1 ("spl: blk: Support loading images from fs")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mayuresh Chitale <mchitale@ventanamicro.com>