driver core: fix smatch warning on dev->bus check

Commit d42a09802174 (driver core: skip removal test for non-removable
drivers) introduced a smatch warning:

drivers/base/dd.c:386 really_probe()
         warn: variable dereferenced before check 'dev->bus' (see line 373)

Fix the warning by removing the dev->bus NULL check. dev->bus will never
be NULL, so the check was unnecessary.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>