phy: ti: gmii-sel: check of_get_address() for failure

Smatch complains that if of_get_address() returns NULL, then "size"
isn't initialized.  Also it would lead to an Oops.

Fixes: 7f78322c ("phy: ti: gmii-sel: retrieve ports number and base offset from dt")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Link: https://lore.kernel.org/r/20210914110038.GB11657@kili


Signed-off-by: Vinod Koul <vkoul@kernel.org>