driver: dmamux: use LL_DMAMUX_CHANNEL_X flag to check if channels exists

Use LL_DMAMUX_CHANNEL_x defines instead of DMAMUX_CSR_SOF7x to check
if corresponding LL_DMAMUX_IsActiveFlag_SOx and LL_DMAMUX_ClearFlag_SOx
inline functions exist and should be added to func_ll_is_active_so[]
and func_ll_clear_so[].
The HAL of some socs uses the same flag to decide which registers exist
on a specific soc. And the same defines are used for table_ll_channel[]
initializations.

This is necessary because DMAMUX_CSR_SOF5 and DMAMUX_CSR_SOF6 were
wrongly added in the HALs soc header file for some stm32g0 socs,
therefore without this change some stm32g0 socs couldn't compile.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>