net/macb: increase RX buffer size for GEM

Macb Ethernet controller requires a RX buffer of 128 bytes. It is
highly sub-optimal for Gigabit-capable GEM that is able to use
a bigger DMA buffer. Change this constant and associated macros
with data stored in the private structure.
RX DMA buffer size has to be multiple of 64 bytes as indicated in
DMA Configuration Register specification.

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>