staging: brcm80211: remove duplicate prototype for si_pmu_pllupd

The prototype for this function occurred twice in this header file
so removed one occurrence.

Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>