Input: zforce - remove unnecessary payload data checks

The function zforce_read_packet() reads 2 values (bytes) of payload
header, validates them and then proceeds to read the payload body.
The function stores all these in a u8 buffer.

The PAYLOAD_LENGTH check seems to be trying to detect an overflow error.
However, since we are just reading a u8 value from the buffer, these
checks are unnecessary and we should simply compare against zero.

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>