firewire: core: use guard macro to maintain static packet data for phy configuration

firewire: core: use guard macro to maintain static packet data for phy configuration

The core function provide a kernel API to send phy configuration packet.
Current implementation of the feature uses packet object allocated
statically. The concurrent access to the object is protected by static
mutex.

This commit uses guard macro to maintain the mutex.

Link: https://lore.kernel.org/r/20240805085408.251763-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>