i2c: aspeed: Mask IRQ status to relevant bits

Mask the IRQ status to only the bits that the driver checks. This
prevents excessive driver warnings when operating in slave mode
when additional bits are set that the driver doesn't handle.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Tao Ren <rentao.bupt@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>