net: bcmgenet: Reset RBUF on first open

net: bcmgenet: Reset RBUF on first open

If the RBUF logic is not reset when the kernel starts then there
may be some data left over from any network boot loader. If the
64-byte packet headers are enabled then this can be fatal.

Extend bcmgenet_dma_disable to do perform the reset, but not when
called from bcmgenet_resume in order to preserve a wake packet.

N.B. This different handling of resume is just based on a hunch -
why else wouldn't one reset the RBUF as well as the TBUF? If this
isn't the case then it's easy to change the patch to make the RBUF
reset unconditional.

See: https://github.com/raspberrypi/linux/issues/3850

Signed-off-by: Phil Elwell <phil@raspberrypi.com>