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 <jpinto@synopsys.com>
Tested-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>