dcd/stm32_fsdev: make CH32 ep0_set_type read-modify-write atomic

ep0_set_type() read the EP0 register outside any critical section and
only ep_write() masked the USB IRQ around the store. A USB interrupt
landing between the read and the write (e.g. a new SETUP whose handler
installs the BULK gate) was silently undone when the task resumed and
wrote back its pre-interrupt snapshot with the type forced to CONTROL,
re-exposing the unsolicited EP0 OUT ACK the workaround blocks.

Bracket the whole read-modify-write with fsdev_int_disable/enable when
called with need_exclusive (task context). ISR-context callers pass
false and are unaffected (the ISR cannot preempt itself).

Co-Authored-By: Claude Fable 5 <[email protected]>

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