extmod/nimble: Improve the flow control for l2cap recv path.

If the _IRQ_L2CAP_RECV handler does the actual consumption of the incoming
data (i.e. via l2cap_recvinto), rather than setting a flag for
non-scheduler-context to handle it later, then two things can happen:

- It can starve the VM (i.e. the scheduled task never terminates).  This is
  because calling l2cap_recvinto will empty the rx buffer, which will grant
  more credits to the channel (an HCI command), meaning more data can
  arrive.  This means that the loop in hal_uart.c that keeps reading HCI
  data from the uart and executing NimBLE events as they are created will
  not terminate, preventing other VM code from running.

- There's no flow control (i.e. data will arrive too quickly).  The channel
  shouldn't be given credits until after we return from scheduler context.

It's preferable that no work is done in scheduler/IRQ context.  But to
prevent this being a problem this commit changes l2cap_recvinto so that if
it is called in IRQ context, and the Python handler empties the rx buffer,
then don't grant credits until the Python handler is complete.

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

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