ltq-vmmc: fix ignored-qualifiers warning

ltq-vmmc: fix ignored-qualifiers warning

NULL and fixed addresses are constant, const qualifier are useless.

/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xrx200/drv_vmmc-1.9.0/src/mps/drv_mps_vmmc_ar9.c:47:1: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
   47 | const void (*ifx_bsp_basic_mps_decrypt)(unsigned int addr, int n) = NULL;
      | ^~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xrx200/drv_vmmc-1.9.0/src/mps/drv_mps_vmmc_ar9.c:246:49: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
  246 |                    ifx_bsp_basic_mps_decrypt = (const void (*)(unsigned int, int))0xbf0017c4;
      |                                                 ^~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xrx200/drv_vmmc-1.9.0/src/mps/drv_mps_vmmc_ar9.c:249:49: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
  249 |                    ifx_bsp_basic_mps_decrypt = (const void (*)(unsigned int, int))0xbf001ea4;
      |                                                 ^~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xrx200/drv_vmmc-1.9.0/src/mps/drv_mps_vmmc_ar9.c:252:49: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
  252 |                    ifx_bsp_basic_mps_decrypt = (const void (*)(unsigned int, int))0xbf001f38;
      |                                                 ^~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <robimarko@gmail.com>