i2c: npcm7xx: Allow 255 byte block SMBus transfers

255 byte support has been tested on a npcm750 board

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Reviewed-by: Tali Perry <tali.perry1@gmail.com>
Reviewed-by: Patrick Venture <venture@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>