net: bcmgenet: introduce bcmgenet_[r|t]dma_disable

The bcmgenet_rdma_disable and bcmgenet_tdma_disable functions
are introduced to provide a common method for disabling each
dma and the code is simplified.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250306192643.2383632-11-opendmb@gmail.com


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