octeon_ep: Remove useless casting value returned by vzalloc to structure

coccinelle reports a warning

WARNING: casting value returned by memory allocation
function to (struct octep_rx_buffer *) is useless.

To fix this the useless cast is removed.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/Yx+sr9o0uylXVcOl@playground


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