[PATCH v4] staging: pi433: add comment to rx_lock mutex definition

[Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>]

Checkpatch reports: CHECK: struct mutex definition without comment.
Fix this by documenting what rx_mutex struct is used for in pi433
driver.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
---
v4: wrap changelog comments at 72 columns where possible (Req:
 Greg k-h)
v3: wrote a more succinct desc so it doesn't become out-of-date (Req:
 Greg k-h)
v2: ellaborate on reasons why the mutex lock is used in driver (Req:
 Greg k-h)
v1: https://lore.kernel.org/lkml/20211222093626.GA13332@localhost.localdomain/
---
 drivers/staging/pi433/pi433_if.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/pi433/pi433_if.c b/drivers/staging/pi433/pi433_if.c
index 29bd37669059..68c09fa016ed 100644
--- a/drivers/staging/pi433/pi433_if.c
+++ b/drivers/staging/pi433/pi433_if.c
@@ -92,7 +92,7 @@ struct pi433_device {
 	u32			rx_bytes_to_drop;
 	u32			rx_bytes_dropped;
 	unsigned int		rx_position;
-	struct mutex		rx_lock;
+	struct mutex		rx_lock; /* protects rx_* variable accesses */
 	wait_queue_head_t	rx_wait_queue;
 
 	/* fifo wait queue */
-- 
2.25.4