i2c-bcm2835: Implement I2C_M_IGNORE_NAK

Now that transfers aren't aborted immediately (and uncleanly) on
errors, and the FIFOs are always drained after all transfers, we
can implement I2C_M_IGNORE_NAK by ignoring the returned error
value.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>