net: mvpp2: cls: Rename MVPP2_N_FLOWS to MVPP2_N_PRS_FLOWS

The macro definition MVPP2_N_FLOWS is ambiguous because it really
represents the number of entries in the Header Parser that are used to
identify the classification flows.

Rename the macro to clearly state that we represent the number of flows
in the Header Parser.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>