EDAC, i82975x: Remove set but not used variable dtype

Fix this gcc -Wunused-but-set-variable warning:

  drivers/edac/i82975x_edac.c:378:16: warning: variable 'dtype'
  	set but not used [-Wunused-but-set-variable]

It was introduced in

  084a4fcc ("edac: move dimm properties to struct dimm_info")

but never used.

Also, remove the function i82975x_dram_type() and move the comment and
the assignment to the place where it is used.

 [ bp: massage commit message and shorten comment. ]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
CC: "Arvind R." <arvino55@gmail.com>
CC: Mauro Carvalho Chehab <mchehab@kernel.org>
CC: ravi@jetztechnologies.com
CC: arvino55@gmail.com
CC: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20181107022237.14048-1-yuehaibing@huawei.com