RISC-V: selftests: Convert hwprobe test to kselftest API

Returning (exiting with) negative exit codes isn't user friendly,
because the user must output the exit code with the shell, convert it
from its unsigned 8-bit value back to the negative value, and then
look up where that comes from in the code (which may be multiple
places). Use the kselftests TAP interface, instead.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230918131518.56803-13-ajones@ventanamicro.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>