mfd: Fix compile errors and warnings when !CONFIG_AB8500_BM

drivers/mfd/ab8500-core.c:1015:21: error: ‘ab8500_bm_data’ undeclared here

include/linux/mfd/abx500/ab8500-bm.h:445:13: warning: ‘ab8500_fg_reinit’ defined but not used
include/linux/mfd/abx500/ab8500-bm.h:448:13: warning: ‘ab8500_charger_usb_state_changed’ defined but not used
include/linux/mfd/abx500/ab8500-bm.h:451:29: warning: ‘ab8500_btemp_get’ defined but not used
include/linux/mfd/abx500/ab8500-bm.h:455:12: warning: ‘ab8500_btemp_get_batctrl_temp’ defined but not used
include/linux/mfd/abx500/ab8500-bm.h:463:12: warning: ‘ab8500_fg_inst_curr_blocking’ defined but not used
include/linux/mfd/abx500/ab8500-bm.h:442:12: warning: ‘ab8500_fg_inst_curr_done’ defined but not used
include/linux/mfd/abx500/ab8500-bm.h:447:26: warning: ‘ab8500_fg_get’ defined but not used

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>