risc-v/mpfs: usb: fix fierce cpu polling if remote closes #224
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If the remote end just closes an endpoint and no longer handles it, the system is prone to intensive cpu polling via mpfs_write_tx_fifo() especially if the device side doesn't know a thing about what the remote did.
Fix this by marking the EP as dead, which will skip all writes causing unnecessary polling. The EP is back in business if the remote end sends some data (rx) or the connection is re-established.