The number of RX and TX buffers for the driver are controlled by BSPCFG_RX_RING_LEN and BSPCFG_TX_RING_LEN. You can lower these by overriding the default values. FOr example, add :
#define BSPCFG_RX_RING_LEN 3
#define BSPCFG_TX_RING_LEN 3
to user_config.h and rebuild your BSP.