i2c: xlp9xx: Make sure the transfer size is not more than I2C_SMBUS_BLOCK_SIZE

For SMBus transactions the max permissible transfer size is
I2C_SMBUS_BLOCK_SIZE. It is possible that some clients might
not follow it strictly occasionally.
This would lead to stack corruption if the driver copies more than
I2C_SMBUS_BLOCK_SIZE bytes. Add a check to avoid such conditions.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: George Cherian <george.cherian@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>