usb: phy: qusb: Don't try to ioremap NULL resource

platform_get_resource_byname() returns resource as NULL if it doesn't
find requested resource. If it is NULL, the subsequent devm_ioremap()
will print an "invalid resource" message to console which can be
misleading. Since these resources are optional, avoid the false error
message by only calling devm_ioremap() if the resources are found.

Change-Id: Ib696958085634b3593d99f124e54fc2216d0f292
Signed-off-by: Mayank Rana <mrana@codeaurora.org>