mfd: bxtwc: Remove unnecessary i2c_addr checks in ipc calls

In the following code block, BXTWC_DEVICE1_ADDR value is
already fixed and hence there no need to check for
if (!i2c_addr) in every ipc read/write calls. Even if this
check is required it can be moved to probe function.

i2c_addr = BXTWC_DEVICE1_ADDR;
if (!i2c_addr) {
	dev_err(pmic->dev, "I2C address not set\n");
	return -EINVAL;
}

This patch remove this extra check and adds some NULL
parameter checks.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>