ANDROID: usb: gadget: android_f_accessory: Assign NULL on misc_register failure

Currently, misc_register fails in acc_init, leading to an error
condition that calls kfree for acc_dev. However, the composite driver
calls android_acc_disconnect regardless of whether acc_init was
successful or not. Since the accessory driver uses acc_dev_instance as a
global pointer, android_acc_disconnect proceeds with a stale dev
variable if acc_dev_instance is not NULL. This results in an after-free
error when accessing the kref of dev.

To fix this, assign acc_dev_instance to NULL. This ensures that when
get_acc_dev is called, the NULL check prevents the extraction of the
kref from a stale dev variable. Also protect the failure path with
acc_dev_instance_lock since multiple threads could try to access the
acc_dev_instance.

Bug: 373331241
Change-Id: Ib8512bd8855f40c99c844b56c58f69d8d3c9ee94
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>