leds: sun50i-a100: Avoid division-by-zero warning

leds: sun50i-a100: Avoid division-by-zero warning

When CONFIG_COMMON_CLK is disabled, e.g. on an x86 randconfig compile test,
clang reports a field overflow from propagating the result of a division by
zero:

drivers/leds/leds-sun50i-a100.c:309:12: error: call to '__compiletime_assert_265' declared with 'error' attribute: FIELD_PREP: value too large for the field
        control = FIELD_PREP(LEDC_T01_TIMING_CTRL_REG_T1H, timing->t1h_ns / cycle_ns) |

Avoid the problem by adding an explicit check for the zero value here. Alternatively
the assertion could be avoided with a Kconfig dependency on COMMON_CLK.

Fixes: 090a25ad9798 ("leds: sun50i-a100: New driver for the A100 LED controller")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20231212214536.175327-1-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>