Bluetooth: hci_uart: prevent kernel crash due to race condition

Bluetooth: hci_uart: prevent kernel crash due to race condition

There is a race condition during tty close between
cancel_work_sync and hci_uart_tx_wakeup where
hci_uart_write_work may still be scheduled after call
to cancel_work_sync which causes kernel null reference
crash as hci_uart_tty_close frees the required resources

locking out hci_uart_tty_close, hci_uart_tx_wakeup and
work queue along with new flag HCI_UART_PROTO_READY
will prevent kernel crashes and race condition

Bug 200373122

Change-Id: If7c788a934ebf11a1498364db527c850a3105409
Reviewed-on: https://git-master.nvidia.com/r/1637439
GVS: Gerrit_Virtual_Submit
Reviewed-by: Dipen Patel <dipenp@nvidia.com>
Tested-by: Dipen Patel <dipenp@nvidia.com>
Reviewed-by: Winnie Hsu <whsu@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>