driver core: use *switch* statement in really_probe()

There are series of comparisons of the 'ret' variable on the failure path of
really_probe(),  so the  *switch* statement  seems  more appropriate there.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>