dpaa2-eth: fix the pointer passed to PTR_ALIGN on Tx path

The blamed commit increased the needed headroom to account for
alignment. This means that the size required to always align a Tx buffer
was added inside the dpaa2_eth_needed_headroom() function. By doing
that, a manual adjustment of the pointer passed to PTR_ALIGN() was no
longer correct since the 'buffer_start' variable was already pointing
to the start of the skb's memory.

The behavior of the dpaa2-eth driver without this patch was to drop
frames on Tx even when the headroom was matching the 128 bytes
necessary. Fix this by removing the manual adjust of 'buffer_start' from
the PTR_MODE call.

Closes: https://lore.kernel.org/netdev/70f0dcd9-1906-4d13-82df-7bbbbe7194c6@app.fastmail.com/T/#u
Fixes: f422abe3f23d ("dpaa2-eth: increase the needed headroom to account for alignment")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Tested-by: Mathew McBride <matt@traverse.com.au>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251016135807.360978-1-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>