Indentation should be done with spaces, not with TABs, in the net subsystem

There are still a lot of old files in the QEMU networking code base that use TABs for indentation instead of using spaces in these files here:

 hw/net/ne2000.c
 hw/net/mcf_fec.c
 hw/net/pcnet.c
 hw/net/can/ctu_can_fd_frame.h
 hw/net/can/ctu_can_fd_regs.h
 hw/net/e1000_regs.h
 hw/net/pcnet.h
 net/tap-linux.h

The TABs should be replaced with spaces, to make sure that we have a consistent coding style with an indentation of 4 spaces everywhere.