i2c: mlxcpld: Decrease polling time for performance improvement

Decrease polling time 'MLXCPLD_I2C_POLL_TIME' from 2000 usec to 400
usec. It greatly improves performance of I2C transactions.

Reliability of setting polling time to 400 usec has been thoroughly
validated across all the supported systems.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>