staging: brcm80211: remove unused function si_pmu_ilp_clock()

The function is not used in the driver and has been removed.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>