lms: flush_recv: alloc buf on stack instead of heap
lms: flush_recv: alloc buf on stack instead of heap

No need to use the heap here since buffer is only used as a temporary
trash. Using the stack is quicker.

Change-Id: Iede8dc0903ee3865a52c8e2fd811bcde444fee33

osmocom/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx