Bluetooth: btmtk: Fix wait_on_bit_timeout interruption during shutdown

During the shutdown process, an interrupt occurs that
prematurely terminates the wait for the expected event.
This change replaces TASK_INTERRUPTIBLE with
TASK_UNINTERRUPTIBLE in the wait_on_bit_timeout call to ensure
the shutdown process completes as intended without being
interrupted by signals.

Fixes: d019930b ("Bluetooth: btmtk: move btusb_mtk_hci_wmt_sync to btmtk.c")
Signed-off-by: Jiande Lu <jiande.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>