net: fec: build fec.c and fec_ptp.c to one module

fec_ptp.ko can't run individually
rename fec.c to fec_main.c
Build fec.o and fec_ptp.o into one fec.ko
Remove unnessary EXPORT_SYMBOL in fec_ptp

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>