usb: dwc2: remove no longer needed init_mutex

init_mutex is a leftover from the time, when s3c-hsotg driver did not
implement proper pull up/down control and emulated it by enabling
enabling/disabling usb phy. Proper pull up/down control has been added
by commit 5b9451f8

 ("usb: dwc2: gadget:
use soft-disconnect udc feature in pullup() method"), so init_muxtex can
be removed now to avoid potential deadlocks with other locks.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Tested-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>