net: vertexcom: mse102x: Fix LEN_MASK

The LEN_MASK for CMD_RTS doesn't cover the whole parameter mask.
The Bit 11 is reserved, so adjust LEN_MASK accordingly.

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/20250430133043.7722-3-wahrenst@gmx.net


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