ltq-adsl-mei: fix missing-prototypes warnings

ltq-adsl-mei: fix missing-prototypes warnings

* Mark some functions as static.
* Add missing prototypes.

This patch fixes the following build warnings:

/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-adsl-mei-danube/ltq-adsl-mei/drv_mei_cpe.c:1182:1: error: no previous prototype for 'makeCMV' [-Werror=missing-prototypes]
 1182 | makeCMV (u8 opcode, u8 group, u16 address, u16 index, int size, u16 * data, u16 *CMVMSG)
      | ^~~~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-adsl-mei-danube/ltq-adsl-mei/drv_mei_cpe.c:1855:1: error: no previous prototype for 'DSL_BSP_ATMLedCBUnregister' [-Werror=missing-prototypes]
 1855 | DSL_BSP_ATMLedCBUnregister (int (*ifx_adsl_ledcallback) (void))
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-adsl-mei-danube/ltq-adsl-mei/drv_mei_cpe.c:2448:1: error: no previous prototype for 'IFX_MEI_Ioctls' [-Werror=missing-prototypes]
 2448 | IFX_MEI_Ioctls (DSL_DEV_Device_t * pDev, int from_kernel, unsigned int command, unsigned long lon)
      | ^~~~~~~~~~~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-adsl-mei-danube/ltq-adsl-mei/drv_mei_cpe.c:2699:5: error: no previous prototype for 'ifx_mei_atm_led_blink' [-Werror=missing-prototypes]
 2699 | int ifx_mei_atm_led_blink(void)
      |     ^~~~~~~~~~~~~~~~~~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-adsl-mei-danube/ltq-adsl-mei/drv_mei_cpe.c:2705:5: error: no previous prototype for 'ifx_mei_atm_showtime_check' [-Werror=missing-prototypes]
 2705 | int ifx_mei_atm_showtime_check(int *is_showtime, struct port_cell_info *port_cell, void **xdata_addr)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~

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