i2c-au1550: Fix a misused register problem

Fix a "mis-used register" problem on the AMD MIPS Alchemy au1550
I2C interface.

In summary, the programmable serial controller seems to hang the kernel
when I send a single 'address' byte on the I2C bus.  The patch
essentially uses the PSC_SMBSTAT register's TE (transmit FIFO empty)
bit to check when the transmit FIFO is empty, instead of using the
PSC_SMBEVNT register's TU (transmit underflow) bit.  Using the TE bit
fixed the hang problem.

Signed-off-by: Chris David <cd@chrisdavid.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>