drivers: ethernet: stm32: fix multicast filter configuration

With CONFIG_ETH_STM32_MULTICAST_FILTER=n, MACFFR was being set to 0 instead
of ETH_MULTICASTFRAMESFILTER_NONE, blocking all multicast traffic.

Instead, reset all the relevant bits first, then set either
ETH_MULTICASTFRAMESFILTER_HASHTABLE or ETH_MULTICASTFRAMESFILTER_NONE
depending on Kconfig.

This issue was introduced in #53850, `git diff a5f9fc23~2 a5f9fc23` (the
total diff of that PR) shows what happened - the
`tmp |= ETH_MULTICASTFRAMESFILTER_NONE;` line was removed completely
instead of being gated by `!defined(CONFIG_ETH_STM32_MULTICAST_FILTER)`.

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>