usb: bdc: Fix unused assignment in bdc_probe()

Delete unused initialized value of 'ret', because it will
be assigned by the function clk_prepare_enable().

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>