acpi/nfit: Fix bus command validation

Commit 11189c10 "acpi/nfit: Fix command-supported detection" broke
ND_CMD_CALL for bus-level commands. The "func = cmd" assumption is only
valid for:

    ND_CMD_ARS_CAP
    ND_CMD_ARS_START
    ND_CMD_ARS_STATUS
    ND_CMD_CLEAR_ERROR

The function number otherwise needs to be pulled from the command
payload for:

    NFIT_CMD_TRANSLATE_SPA
    NFIT_CMD_ARS_INJECT_SET
    NFIT_CMD_ARS_INJECT_CLEAR
    NFIT_CMD_ARS_INJECT_GET

Update cmd_to_func() for the bus case and call it in the common path.

Fixes: 11189c10 ("acpi/nfit: Fix command-supported detection")
Cc: <stable@vger.kernel.org>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Reported-by: Grzegorz Burzynski <grzegorz.burzynski@intel.com>
Tested-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>