net: fec: simplify loop counter handling in swap_buffer()

Eliminate the DIV_ROUND_UP() and change the loop counter increment to
4 instead. This results in saving 6 instructions in the functions
assembly code.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: David S. Miller <davem@davemloft.net>