hw/ppc/ppc440_uc.c: Remove incorrect iothread locking from dcr_write_pcie()

hw/ppc/ppc440_uc.c: Remove incorrect iothread locking from dcr_write_pcie()

In dcr_write_pcie() we take the iothread lock around a call to
pcie_host_mmcfg_udpate().  This is an incorrect attempt to deal with
the bug fixed in commit 235352ee6e73d7716, where we were not taking
the iothread lock before calling device dcr read/write functions.
(It's not sufficient locking, because although the other cases in the
switch statement won't assert, there is no locking which prevents
multiple guest CPUs from trying to access the PPC460EXPCIEState
struct at the same time and corrupting data.)

Unfortunately with commit 235352ee6e73d7716 we are now trying
to recursively take the iothread lock, which will assert:

  $ qemu-system-ppc -M sam460ex --display none
  **
  ERROR:/home/petmay01/linaro/qemu-from-laptop/qemu/cpus.c:1830:qemu_mutex_lock_iothread_impl: assertion failed: (!qemu_mutex_iothread_locked())
  Aborted (core dumped)

Remove the locking within dcr_write_pcie().

Fixes: 235352ee6e73d7716
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200330125228.24994-1-peter.maydell@linaro.org>
Tested-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>