i2c: designware: remove redundant lock

The per adapter bus_lock already projects from concurrent calls to the
master_xfer callback. No need to add a driver internal lock.

Also, rephrase a comment to drop mention of this lock.

Reported-by: Rongrong Zou <zourongrong@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>