firmware: google: fix a NULL vs IS_ERR() check in cbmem_entry_probe()

The devm_memremap() function returns error pointers on error,
it doesn't return NULL.

Fixes: 19d54020 ("firmware: google: Implement cbmem in sysfs driver")
Signed-off-by: Peng Wu <wupeng58@huawei.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Link: https://lore.kernel.org/r/20221115091138.51614-1-wupeng58@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>