liquidio: Resolved mbox read issue while reading more than one 64bit data

Corrected length check when data received in the mbox is more than one
64 bit data value

Signed-off-by: Intiyaz Basha <intiyaz.basha@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>