scx200_acb: Fix the block transactions

The scx200_acb i2c bus driver pretends to support SMBus block
transactions, but in fact it implements the more simple I2C block
transactions. Additionally, it lacks sanity checks on the length
of the block transactions, which could lead to a buffer overrun.

This fixes an oops reported by Alexander Atanasov:
http://marc.theaimsgroup.com/?l=linux-kernel&m=114970382125094



Thanks to Ben Gardner for fixing my bugs :)

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>