staging: gdm72xx: use %pI4 format to print IPv4 address and remove last usage of NIP6

commit cf4ca487 removed the definition of NIPQUAD and NIPQUAD_FMT,
and NIP6 also is out of date.

commit 2874762b replace deprecated NIPQUAD marco to C code, but we can use %pI4 to
print IPv4 address more simply. And remove constant condition judge.

Because DEBUG_SDU is not defined in gdm_wimax.h, no error message when compiling.

Signed-off-by: Shan Wei <davidshan@tencent.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>