i2c: aspeed: fixed potential null pointer dereference

Before I skipped null checks when the master is in the STOP state; this
fixes that.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Fixes: f327c686 ("i2c: aspeed: added driver for Aspeed I2C")