i2c: tegra: Use threaded interrupt

Switch to use threaded interrupt context in order to avoid checking of
"are we in interrupt?" for the code that may sleep in the IRQ handler.
I2C doesn't require a very low interrupt-handling latency, hence this
change doesn't introduce any noticeable effects.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>