ravb: Correct buffer size to map for R-Car Rx

When creating a helper to allocate and align an skb one location where
the skb data size was updated was missed. This can lead to a warning
being printed when the memory is being unmapped as it now always unmap
the maximum frame size, instead of the size after it have been
aligned.

This was correctly done for RZ/G2L but missed for R-Car.

Fixes: cfbad647

 ("ravb: Create helper to allocate skb and align it")
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240308224237.496924-1-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Jakub Kicinski <kuba@kernel.org>