A frame is a u8 array with the following structure:
[PAYLOAD_HEADER, PAYLOAD_LENGTH, ...PAYLOAD_BODY...]

PAYLOAD_BODY can be at most 255 bytes long, as it's size is represented
by PAYLOAD_LENGTH. Therefore we can reduce the stack memory allocated to
payload_buffer[] roughly by half, from 512 to 257 bytes.
Signed-off-by: Luis Ortega <luiorpe1@upv.es>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de> - bq Cervantes (imx6sl)
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>