nfc: pn533: Fix use-after-free bugs caused by pn532_cmd_timeout

In the Linux kernel, the following vulnerability has been resolved:

nfc: pn533: Fix use-after-free bugs caused by pn532_cmd_timeout

When the pn532 uart device is detaching, the pn532_uart_remove()

is called. But there are no functions in pn532_uart_remove() that

could delete the cmd_timeout timer, which will cause use-after-free

bugs. The process is shown below:

(thread 1) | (thread 2)

| pn532_uart_send_frame

pn532_uart_remove | mod_timer(&pn532->cmd_timeout,...)

... | (wait a time)

kfree(pn532) //FREE | pn532_cmd_timeout

| pn532_uart_send_frame

| pn532->... //USE

This patch adds del_timer_sync() in pn532_uart_remove() in order to

prevent the use-after-free bugs. What's more, the pn53x_unregister_nfc()

is well synchronized, it sets nfc_dev->shutting_down to true and there

are no syscalls could restart the cmd_timeout timer.