EDAC: Add helper which returns the loaded platform driver

EDAC: Add helper which returns the loaded platform driver

Only a single EDAC platform driver can be loaded. When ghes_edac is
enabled, an EDAC platform driver still attempts to register itself and
fails in edac_mc_add_mc().

Add edac_get_owner() so that EDAC platform drivers can check the owner
first.

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20170823225447.15608-5-toshi.kani@hpe.com
[ Massage commit message. ]
Signed-off-by: Borislav Petkov <bp@suse.de>