net: dsa: motorcomm: Fix register bit field names

Review and fix the incorrectness in the bit field names. No functional
change.

Signed-off-by: David Yang <mmyangfl@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260923021945.2203368-5-mmyangfl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>