1. The issue concerns sdmmc library bug (I finally solved) in SD_Init() in the file sdmmc/src/fsl_sd.c:SD_SelectBusTiming()
calls SD_SwitchFunction() which sets block size to 64bytes (512bits).Therefore SD_SetBlockSize(card, FSL_SDMMC_DEFAULT_BLOCK_SIZE)
should be called again before SD_Init() exits.
2. The issue is wrong variable in drv_emac.c:lpc_emac_tx(), and the wrong data length of send packet
to cause the send failure.
Signed-off-by: Nzheng-chow <sernia.zhou@foxmail.com>

1. Change the type of cmd to 'int';
2. Remove RT_LWIP_USING_RT_MEM macro;