i2c: bcm2835: Support i2c-dev ioctl I2C_TIMEOUT

Use i2c_adapter->timeout for the completion timeout value. The core
default is 1 second.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>