Revert "EmbeddedPkg/Lan9118Dxe: use MemoryFence"

Revert "EmbeddedPkg/Lan9118Dxe: use MemoryFence"

Commit a4626006bbf86113 ("EmbeddedPkg/Lan9118Dxe: use MemoryFence")
replaced some stalls with memory fences, on the presumption that these
were erroneously being used to order memory accesses. However, this was
not the case.

LAN9118 devices require a timing delay between state-changing
reads/writes and subsequent reads, as updates to the register file are
asynchronous and the effects of state-changes are not immediately
visible to subsequent reads.

This delay cannot be ensured through the use of memory barriers, which
only enforce observable ordering, and not timing. Thus, converting these
stalls to memory fences was erroneous, and may result in stale values
being read.

This reverts commit a4626006bbf86113453aeb7920895e66cdd04737.

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>