Bluetooth: btbcm: Fix NULL deref in btbcm_get_board_name()

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: btbcm: Fix NULL deref in btbcm_get_board_name() devm_kstrdup() can return a NULL pointer on failure,but this returned value in btbcm_get_board_name() is not checked. Add NULL check in btbcm_get_board_name(), to handle kernel NULL pointer dereference error.