Replace risky DEBUGASSERT()s with real sanity checks. Also,
do a few more checks as the system might occasionally fire an
interrupt if the system has been restarted while in middle of
an i2c transaction.
Yet, modify i2c_transfer() function so that up_disable_irq()
is always called at the end to better prevent ill-timed irqs.
Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>