r8169: Fix spelling mistake: "tx_underun" -> "tx_underrun"

r8169: Fix spelling mistake: "tx_underun" -> "tx_underrun"

[ Upstream commit 8df9439389a44fb2cc4ef695e08d6a8870b1616c ]

There is a spelling mistake in the struct field tx_underun, rename
it to tx_underrun.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/20240909140021.64884-1-colin.i.king@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: ced8e8b8f40a ("r8169: add tally counter fields added with RTL8125")
Signed-off-by: Sasha Levin <sashal@kernel.org>