drm/i915/cnl: Fix _CNL_PORT_TX_DW2_LN0_F definition.

This was wrong since its introduction on commit '04416108
("drm/i915/cnl: Add registers related to voltage swing sequences.")'

But since no Port F was needed so far we don't need to
propagate fixes back there.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180129232223.766-4-rodrigo.vivi@intel.com