ipheth: fix EOVERFLOW in ipheth_rcvbulk_callback

When rx_buf is allocated we need to account for IPHETH_IP_ALIGN,
which reduces the usable size by 2 bytes. Otherwise we have 1512
bytes usable instead of 1514, and if we receive more than 1512
bytes, ipheth_rcvbulk_callback is called with status -EOVERFLOW,
after which the driver malfunctiones and all communication stops.

Resolves ipheth 2-1:4.2: ipheth_rcvbulk_callback: urb status: -75

Fixes: f33d9e2b

 ("usbnet: ipheth: fix connectivity with iOS 14")
Signed-off-by: Georgi Valkov <gvalkov@abv.bg>
Tested-by: Jan Kiszka <jan.kiszka@siemens.com>
Link: https://lore.kernel.org/all/B60B8A4B-92A0-49B3-805D-809A2433B46C@abv.bg/
Link: https://lore.kernel.org/all/24851bd2769434a5fc24730dce8e8a984c5a4505.1643699778.git.jan.kiszka@siemens.com/


Signed-off-by: Jakub Kicinski <kuba@kernel.org>