ravb: Consolidate clock handling

The module clock is used for two purposes:
  - Wake-on-LAN (WoL), which is optional,
  - gPTP Timer Increment (GTI) configuration, which is mandatory.

As the clock is needed for GTI configuration anyway, WoL is always
available.  Hence remove duplication and repeated obtaining of the clock
by making GTI use the stored clock for WoL use.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>