net: lan966x: Fix FDMA when MTU is changed

When MTU is changed, FDMA is required to calculate what is the maximum
size of the frame that it can received. So it can calculate what is the
page order needed to allocate for the received frames.
The first problem was that, when the max MTU was calculated it was
reading the value from dev and not from HW, so in this way it was
missing L2 header + the FCS.
The other problem was that once the skb is created using
__build_skb_around, it would reserve some space for skb_shared_info.
So if we received a frame which size is at the limit of the page order
then the creating will failed because it would not have space to put all
the data.

Fixes: 2ea1cbac ("net: lan966x: Update FDMA to change MTU.")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>