kernel: ltq-ptm: fix compilation warning for vr9

kernel: ltq-ptm: fix compilation warning for vr9

Fix compilation warning for unused variable for vr9.
Fix compilation warning:
/home/ansuel/openwrt-ansuel/openwrt/build_dir/target-mips-openwrt-linux-musl_musl/linux-lantiq_xrx200/ltq-ptm-vr9/ltq-ptm/ifxmips_ptm_vdsl.c: In function 'mailbox_irq_handler':
/home/ansuel/openwrt-ansuel/openwrt/build_dir/target-mips-openwrt-linux-musl_musl/linux-lantiq_xrx200/ltq-ptm-vr9/ltq-ptm/ifxmips_ptm_vdsl.c:560:9: error: unused variable 'i' [-Werror=unused-variable]
  560 |     int i;
      |         ^
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>