i2c: iproc: update slave isr mask (ISR_MASK_SLAVE)

Update slave isr mask (ISR_MASK_SLAVE) to include remaining
two slave interrupts.

Fixes: c245d94e

 ("i2c: iproc: Add multi byte read-write support for slave mode")
Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Acked-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>