EDAC/mc: Determine mci pointer from the error descriptor

Each struct mci has its own error descriptor. Create a function
error_desc_to_mci() to determine the corresponding mci from an
error descriptor. This removes @mci from the parameter list of
edac_raw_mc_handle_error() as the mci pointer does not need to be passed
any longer.

 [ bp: Massage commit message. ]

Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Aristeu Rozanski <aris@redhat.com>
Link: https://lkml.kernel.org/r/20200123090210.26933-5-rrichter@marvell.com