lantiq: fix vmmc driver build warnings

lantiq: fix vmmc driver build warnings

This patch fixes the following build warnings:

arch/mips/lantiq/xway/vmmc.c:31:14: error: no previous prototype for 'ltq_get_mps_ad0_irq' [-Werror=missing-prototypes]
   31 | unsigned int ltq_get_mps_ad0_irq(void)
      |              ^~~~~~~~~~~~~~~~~~~
arch/mips/lantiq/xway/vmmc.c:40:14: error: no previous prototype for 'ltq_get_mps_ad1_irq' [-Werror=missing-prototypes]
   40 | unsigned int ltq_get_mps_ad1_irq(void)
      |              ^~~~~~~~~~~~~~~~~~~
arch/mips/lantiq/xway/vmmc.c:49:14: error: no previous prototype for 'ltq_get_mps_vc_irq' [-Werror=missing-prototypes]
   49 | unsigned int ltq_get_mps_vc_irq(int idx)
      |              ^~~~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <[email protected]>