net: Implement VLAN priority to packet priority conversion

According to IEEE 802.1Q the VLAN priority (PCP) is not directly mapped
to the network packet priority. The Best Effort priority has a PCP value
of 0. The lowest priority (Background) has a PCP value of 1.

All the values are mapped according to the following table:

  +-----+-----+---------+
  | PCP | PRI | Acronym |
  +-----+-----+---------+
  |  1  |  0  |    BK   |
  |  0  |  1  |    BE   |
  |  2  |  2  |    EE   |
  |  3  |  3  |    CA   |
  |  4  |  4  |    VI   |
  |  5  |  5  |    VO   |
  |  6  |  6  |    IC   |
  |  7  |  7  |    NC   |
  +-----+-----+---------+

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>