net: marvell: mvpp2: Fix the computation of shared CPUs

'bitmap_fill()' fills a bitmap one 'long' at a time.
It is likely that an exact number of bits is expected.

Use 'bitmap_set()' instead in order not to set unexpected bits.

Fixes: e531f767 ("net: mvpp2: handle cases where more CPUs are available than s/w threads")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>