net: mvneta: Do not allocate buffer in rxq init with HWBM

For HWBM all buffers are allocated in mvneta_bm_construct() and in runtime
they are put into descriptors by hardware. There is no need to fill them
at this point.

Suggested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>