i2c-au1550: increase timeout waiting for master done

Double the timeout in the loop which busy-waits for the "master-done"
bit to be set.  This bit indicates whether an i2c transaction has
completed;  on the DB1300 and DB1550 boards this timeout is slightly
too short and causes transactions to the WM8731 codec to be falsely flagged
as failed.  The timeout itself is necessary since transactions to
non-existant slaves never set this bit in the first place (and cause
i2cdetect to hang).

With this change the WM8731 codec on the DB1300/DB1550 boards is correctly
detected and initialized.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>