i2c: meson: improve interrupt handler and detect spurious interrupts

If state is STATE_IDLE no interrupt should occur. Return IRQ_NONE
if such a spurious interrupt is detected.
Not having to take care of STATE_IDLE later in the interrupt handler
allows to further simplify the interrupt handler in subsequent
patches of this series.

In addition move resetting REG_CTRL_START bit to the start of the
interrupt handler to ensure that the start bit is always reset.
Currently the start bit is not reset for STATE_STOP because
i2c->state is set to STATE_IDLE.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>