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.

Signed-off-by: Mark Hasemeyer <markhas@chromium.org>
Link: https://lore.kernel.org/r/20231107144743.v1.1.I7987f05f61901f567f7661763646cb7d7919b528@changeid


Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@kernel.org