net: stmmac: break some functions into RX and TX scopes

This patch breaks several functions into RX and TX scopes, which
will be useful when adding multiple buffers mechanism.

Signed-off-by: Joao Pinto <[email protected]>
Tested-by: Niklas Cassel <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Joao Pinto