net: ravb: Split GTI computation and set operations

ravb_set_gti() was computing the value of GTI based on the reference clock
rate and then applied it to register. This was done on the driver's probe
function. In order to implement runtime PM for all IP variants (as some IP
variants switches to reset mode (and thus the registers content is lost)
when module standby is configured through clock APIs) the GTI setup was
split in 2 parts: one computing the value of the GTI register (done in the
driver's probe function) and one applying the computed value to register
(done in the driver's ndo_open API).

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>