i2c-bfin-twi: Remove useless twi_lock mutex

Remove this unneeded mutex. Indeed it was used to serialize access to
the hardware, but this is already done by the i2c-core layer, see
'bus_lock' mutex used by i2c_transfer().

Signed-off-by: Francis Moreau <francis.moro@gmail.com>
Acked-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>