Revert "In linc_us4_packet:set_field, values are always whole bytes"

This reverts commit 2ba318d620254ebe2cebf3c7c3447ca66c01dab2.

After introducing padding removal into the decoder all match fields'
values (that are also parts of set_field) are stripped to the correct
size. So it's no longer need to extract bits for ECN, DSCP or VLAN PCP
values.
(Motivated by #144).
Fix setting VLAN ID

After introducing stripping binary values of match fields to their
correct sizes linc_us4_packet:set_field/2 had to be fixed for setting
VLAN ID.
(Motivated by #144).