apps/netcmd: fix compilation errors when CONFIG_NETUTILS_TFTPC=y

apps/netcmd: fix compilation errors when CONFIG_NETUTILS_TFTPC=y

This patch fixes following compilation errors:

    netcmd.c: In function 'cmd_get':
    netcmd.c:378:2: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'char'
      char newfilename[30];
      ^
    netcmd.c:389:11: error: assignment makes pointer from integer without a cast [-Werror]
      fullpath = get_fullpath(args.destpath);
               ^
    netcmd.c:407:3: error: 'return' with no value, in function returning non-void [-Werror=return-type]
       return;
       ^
    netcmd.c:379:7: error: unused variable 'seek_wbuffer' [-Werror=unused-variable]
      char seek_wbuffer[100];
           ^
    netcmd.c:378:7: error: unused variable 'newfilename' [-Werror=unused-variable]
      char newfilename[30];
           ^
    netcmd.c:376:7: error: unused variable 'read_buffer' [-Werror=unused-variable]
      char read_buffer[301];
           ^
    netcmd.c:375:7: error: unused variable 'write_buffer' [-Werror=unused-variable]
      char write_buffer[300];
           ^

Change-Id: I6fd0ed45f936d34b5425fb783c31da41739bb247
Signed-off-by: EunBong Song <eunb.song@samsung.com>