ptp: Convert chardev code to lock guards

Convert the various spin_lock_irqsave() protected critical regions to
scoped guards. Use spinlock_irq instead of spinlock_irqsave as all the
functions are invoked in thread context with interrupts enabled.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20250625115133.425029269@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>