usb: bdc: Use devm_clk_get_optional()

The BDC clock is optional and we may get an -EPROBE_DEFER error code
which would not be propagated correctly, fix this by using
devm_clk_get_optional().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>