libnvdimm: remove redundant assignment to pointer 'dev'

Pointer dev is being assigned a value that is never read, it is being
re-assigned the same value later on, hence the initialization is redundant
and can be removed.

Cleans up clang warning:
drivers/nvdimm/pfn_devs.c:307:17: warning: Value stored to 'dev' during
its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>