stm32/rfcore: Don't send HCI ACL cmds while another is pending.
stm32/rfcore: Don't send HCI ACL cmds while another is pending.

And, for TX, the next/prev entries ane unused so set them to NULL to
indicate this.

Signed-off-by: Jim Mussared <e84f5c941266186d0c97dcc873413469b954847e@gmail.com>

bvernoux/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,MrSurly/micropython,bvernoux/micropython,MrSurly/micropython,henriknelson/micropython,MrSurly/micropython,MrSurly/micropython,henriknelson/micropython,MrSurly/micropython,adafruit/circuitpython,henriknelson/micropython,adafruit/circuitpython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,henriknelson/micropython