[C++/mingw] ser-tcp.c casts

[C++/mingw] ser-tcp.c casts

Fixes a few errors like these:

../../src/gdb/ser-tcp.c: In function 'int net_open(serial*, const char*)':
../../src/gdb/ser-tcp.c:286:73: error: invalid conversion from 'void*' to 'char*' [-fpermissive]
     res = getsockopt (scb->fd, SOL_SOCKET, SO_ERROR, (void *) &err, &len);
                                                                         ^

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* ser-tcp.c (net_open) : Cast getsockopt argument to char *
	instead of void *.  Update comment.
	(net_read_prim): Cast recv argument to char * instead of void *.
	(net_write_prim): Cast send argument to char *.  Add comment.