phy: qcom-qusb2: fix missing assignment of ret when calling clk_prepare_enable

The error return from the call to clk_prepare_enable is not being assigned
to variable ret even though ret is being used to check if the call failed.
Fix this by adding in the missing assignment.

Addresses-Coverity: ("Logically dead code")
Fixes: 891a96f6 ("phy: qcom-qusb2: Add support for runtime PM")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>