i2c: ismt: Use minimum descriptor size

i2c: ismt: Use minimum descriptor size

Software is allowed to allocate number of descriptor size from 2 to 256,
this i2c controller could process more descriptor, but for i2c core soft
ware layer, only one i2c transaction is allowed each time.

So here switch to minimum 2 descriptor when initialization.

Signed-off-by: Fan Du <fan.du@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>