i2c: designware: Do not enable interrupts shortly in polling mode

I was testing the polling mode txgbe_i2c_dw_xfer_quirk() on a HW where
the i2c-designware has interrupt connected and shared with other device.
I noticed there is a bogus interrupt for each transfer.

Reason for this that both polling mode functions call the
i2c_dw_xfer_init() which enable interrupts then followed by immediate
disable by the same polling mode functions. This is enough to trigger
TX_EMPTY interrupt.

Fix this by introducing a __i2c_dw_write_intr_mask() helper that unmasks
interrupts conditionally and use it in i2c_dw_xfer_init().

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>