mvpp2: refactor BM pool init percpu code

In mvpp2_swf_bm_pool_init_percpu(), a reference to a struct
mvpp2_bm_pool is obtained traversing multiple structs, when a
local variable already points to the same object.

Fix it and, while at it, give the variable a meaningful name.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>