staging: rts5139: fix bad symbol declaration

With sparse, the following error appears :
  CHECK   drivers/staging/rts5139/ms_mg.c
drivers/staging/rts5139/ms_mg.c:82:5: warning: symbol 'mg_set_tpc_para_sub' was not declared. Should it be static?

Rename function with correct prefix and move declaration to coherent internal header file.

Signed-off-by: Neil 'Superna' Armstrong <superna9999@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>