kernel: ltq-adsl-mei: fix compilation warning for copy_from_user ret

kernel: ltq-adsl-mei: fix compilation warning for copy_from_user ret

Fix compilation warning for copy_from_user ret value not handled.
Fix compilation warning:
/__w/openwrt/openwrt/openwrt/build_dir/target-mips_mips32_musl/linux-lantiq_ase/ltq-adsl-mei-ase/ltq-adsl-mei/drv_mei_cpe.c: In function 'DSL_BSP_FWDownload':
/__w/openwrt/openwrt/openwrt/build_dir/target-mips_mips32_musl/linux-lantiq_ase/ltq-adsl-mei-ase/ltq-adsl-mei/drv_mei_cpe.c:1623:17: error: ignoring return value of 'copy_from_user' declared with attribute 'warn_unused_result' [-Werror=unused-result]
 1623 |                 copy_from_user ((char *) &img_hdr_tmp, buf, sizeof (img_hdr_tmp));
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/__w/openwrt/openwrt/openwrt/build_dir/target-mips_mips32_musl/linux-lantiq_ase/ltq-adsl-mei-ase/ltq-adsl-mei/drv_mei_cpe.c:1701:17: error: ignoring return value of 'copy_from_user' declared with attribute 'warn_unused_result' [-Werror=unused-result]
 1701 |                 copy_from_user (mem_ptr, buf + nRead, nCopy);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

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