drivers/firmware/broadcom/bcm47xx_nvram.c: use __ioread32_copy() instead of open-coding

Now that we have a generic library function for this, replace the
open-coded instance.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: <zajec5@gmail.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>