MIPS: sni: Fix the build

This patch fixes the following gcc 10 build error:

arch/mips/sni/time.c: In function ‘a20r_set_periodic’:
arch/mips/sni/time.c:15:26: error: unsigned conversion from ‘int’ to ‘u8’ {aka ‘volatile unsigned char’} changes value from ‘576’ to ‘64’ [-Werror=overflow]
   15 | #define SNI_COUNTER0_DIV ((SNI_CLOCK_TICK_RATE / SNI_COUNTER2_DIV) / HZ)
      |                          ^
arch/mips/sni/time.c:21:45: note: in expansion of macro ‘SNI_COUNTER0_DIV’
   21 |  *(volatile u8 *)(A20R_PT_CLOCK_BASE + 0) = SNI_COUNTER0_DIV;
      |                                             ^~~~~~~~~~~~~~~~

Cc: linux-mips@vger.kernel.org
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>