net: vertexcom: mse102x: Add range check for CMD_RTS

Since there is no protection in the SPI protocol against electrical
interferences, the driver shouldn't blindly trust the length payload
of CMD_RTS. So introduce a bounds check for incoming frames.

Fixes: 2f207cbf ("net: vertexcom: Add MSE102x SPI support")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250430133043.7722-4-wahrenst@gmx.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>