brcm80211: moved power conversion functions

Moved brcmu_mw_to_qdbm and brcmu_qdbm_to_mw functions into the only
file using them. Names were adjusted accordingly.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>