pcuif_proto: cosmetic: add constant PCU_IF_NUM_NSVC and replace magic numbers
pcuif_proto: cosmetic: add constant PCU_IF_NUM_NSVC and replace magic numbers

The number of NSVCs is fixed but lets not use magic numbers to define
the sizes of the arrays that hold the config values. In osmo-pcu there
is already a define constant, so lets use a define here as well.

Change-Id: If7fa44abb86c18900110d7ee81fe6140c8c4635b

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts