efi_loader: simplify error message in efi_disk_create_raw()

The error message

    Adding disk for usb_mass_storage.lun0 failed (err=-9223372036854775788/0x8000000000000014)

provides a decimal and a hexadecimal notation of the EFI status code
EFI_ALREADY_STARTED which is defined as (EFI_ERROR_MASK | 20).
The decimal output does not convey the value 20 clearly.

With the patch we write

    Adding block device usb_mass_storage.lun0 failed, r = 20

similar to other EFI error messages.

Fixes: 95201811 ("dm: sandbox: Switch over to using the new host uclass")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>