i2c: aspeed: Adjust spinlock scope in the irq handler

This patch adjusts spinlock scope to make it wrap the whole irq
handler using a single lock/unlock which covers both master and
slave handlers.

Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>