r8169: rename r8169.c to r8169_main.c

[ Upstream commit 25e992a4603cd5284127e2a6fda6b05bd58d12ed ]

In preparation of factoring out firmware handling rename r8169.c to
r8169_main.c.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: dcf75a0f6bc1 ("r8169: fix the KCSAN reported data-race in rtl_tx while reading TxDescArray[entry].opts1")
Signed-off-by: Sasha Levin <sashal@kernel.org>