i2c: designware: Use accessors to DW_IC_INTR_MASK register

Convert access to DW_IC_INTR_MASK register using the existing
__i2c_dw_write_intr_mask() and a __i2c_dw_read_intr_mask() introduced
here. Motivation to this is to prepare for generic polling mode code
where polling mode will use a SW mask instead of DW_IC_INTR_MASK.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>