Use lock to guard against concurrent access for soft-connect/disconnect
operations when writing to soft_connect sysfs.

Fixes: 2ccea03a ("usb: gadget: introduce UDC Class")
Cc: stable@vger.kernel.org
Acked-by: NFelipe Balbi <balbi@kernel.org>
Signed-off-by: NThinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/338ea01fbd69b1985ef58f0f59af02c805ddf189.1610611437.git.Thinh.Nguyen@synopsys.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>