driver core: fix documentation of request_firmware_nowait

request_firmware_nowait declares it can be called in non-sleep contexts,
but kthead_run called by request_firmware_nowait may sleep. So fix its
documentation and comment to make callers clear about it.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>