nfit: fix unchecked dereference in acpi_nfit_ctl

Incremental patch to fix the unchecked dereference in acpi_nfit_ctl.
Reported by Dan Carpenter:

"acpi/nfit: fix cmd_rc for acpi_nfit_ctl to
always return a value" from Jun 28, 2018, leads to the following
Smatch complaint:

    drivers/acpi/nfit/core.c:578 acpi_nfit_ctl()
     warn: variable dereferenced before check 'cmd_rc' (see line 411)

drivers/acpi/nfit/core.c
   410
   411		*cmd_rc = -EINVAL;
                ^^^^^^^^^^^^^^^^^^
Patch adds unchecked dereference.

Fixes: c1985cef

 ("acpi/nfit: fix cmd_rc for acpi_nfit_ctl to always return a value")

Signed-off-by: Dave Jiang <dave.jiang@intel.com>