net: mvneta: make stub functions static inline

If the CONFIG_MVNET_BA is not set, then make the stub functions
static inline to avoid trying to export them, and remove hte
following sparse warnings:

drivers/net/ethernet/marvell/mvneta_bm.h:163:6: warning: symbol 'mvneta_bm_pool_destroy' was not declared. Should it be static?
drivers/net/ethernet/marvell/mvneta_bm.h:165:6: warning: symbol 'mvneta_bm_bufs_free' was not declared. Should it be static?
drivers/net/ethernet/marvell/mvneta_bm.h:167:5: warning: symbol 'mvneta_bm_construct' was not declared. Should it be static?
drivers/net/ethernet/marvell/mvneta_bm.h:168:5: warning: symbol 'mvneta_bm_pool_refill' was not declared. Should it be static?
drivers/net/ethernet/marvell/mvneta_bm.h:170:23: warning: symbol 'mvneta_bm_pool_use' was not declared. Should it be static?
drivers/net/ethernet/marvell/mvneta_bm.h:181:18: warning: symbol 'mvneta_bm_get' was not declared. Should it be static?
drivers/net/ethernet/marvell/mvneta_bm.h:182:6: warning: symbol 'mvneta_bm_put' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>