i2c: i2c-mux-gpio: Factor out pdev->dev in _probe_dt()

Factor out &pdev->dev into a local variable in preparation for
the ACPI enablement of this function, which will utilize the variable
more.

Signed-off-by: Evan Green <evgreen@chromium.org>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>