i2c: dev: Handle 255 byte blocks for i2c ioctl

I2C_SMBUS is limited to 32 bytes due to compatibility with the
32 byte i2c_smbus_data.block

I2C_RDWR allows larger transfers if sufficient sized buffers are passed.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>