spi: Fix null dereference on suspend

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

spi: Fix null dereference on suspend

A race condition exists where a synchronous (noqueue) transfer can be

active during a system suspend. This can cause a null pointer

dereference exception to occur when the system resumes.

Example order of events leading to the exception:

1. spi_sync() calls __spi_transfer_message_noqueue() which sets

ctlr->cur_msg

2. Spi transfer begins via spi_transfer_one_message()

3. System is suspended interrupting the transfer context

4. System is resumed

6. spi_controller_resume() calls spi_start_queue() which resets cur_msg

to NULL

7. Spi transfer context resumes and spi_finalize_current_message() is

called which dereferences cur_msg (which is now NULL)

Wait for synchronous transfers to complete before suspending by

acquiring the bus mutex and setting/checking a suspend flag.