SPI TX Buffer size should be optimized

[jeromecoutant]

See #55, buffer size is 2500, whereas ES_WIFI_MAX_TX_PACKET_SIZE is 1460

Issue is due to old wifi FW version in DISCO-F413

Propositions:

• add some FW version check before setting the size?
• make a specific buffer for TX, for RX, and for scan procedure ?