driver core: warn about duplicate driver names on the same bus

Currently an attempt to register multiple
drivers with the same name causes the
stack trace with some cryptic error message.
The attached patch adds the necessary check
and the clear error message.

Signed-off-by: Stas Sergeev <stsp@aknet.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>