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 <redacted>
---
v2: ellaborate on reasons why the mutex lock is used in the driver (Req: Greg k-h)
v1: https://lore.kernel.org/lkml/20211222093626.GA13332@localhost.localdomain/ (local)
---
 drivers/staging/pi433/pi433_if.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/staging/pi433/pi433_if.c b/drivers/staging/pi433/pi433_if.c
index 29bd37669059..1cd3d5f2df2a 100644
--- a/drivers/staging/pi433/pi433_if.c
+++ b/drivers/staging/pi433/pi433_if.c

@@ -92,6 +92,17 @@ struct pi433_device {
 	u32			rx_bytes_to_drop;
 	u32			rx_bytes_dropped;
 	unsigned int		rx_position;
+	/*
+	 * rx_lock is used to avoid race-conditions that can be triggered from userspace.
+	 *
+	 * For instance, if a program in userspace is reading the char device
+	 * allocated in this module then another program won't be able to change RX
+	 * configuration of the RF69 hardware module via ioctl and vice versa.
+	 *
+	 * utilization summary:
+	 *  - pi433_read: blocks are read until rx read something (up to the buffer size)
+	 *  - pi433_ioctl: during pending read request, change of config not allowed
+	 */