staging: rts5139: fixed issues when config to built-in object

Fixed two issues when CONFIG_RTS5139=y :

  - Makefile doesn't take $(CONFIG_RTS5139). It always uses obj-m and built as
    a loadable module.
  - Rename some symbols with prefix 'rts51x_' to prevent symbol name collisions
    with drivers/staging/rts_pstor when both are configured to be built-in
    objects.

  drivers/staging/rts5139/built-in.o: In function `xd_cleanup_work':
  (.text+0x1435d): multiple definition of `xd_cleanup_work'
  drivers/staging/rts_pstor/built-in.o:(.text+0x2b96a): first defined here
  drivers/staging/rts5139/built-in.o: In function `release_xd_card':
  (.text+0x14393): multiple definition of `release_xd_card'
  drivers/staging/rts_pstor/built-in.o:(.text+0x2c491): first defined here
  drivers/staging/rts5139/built-in.o: In function `set_sense_data':
  (.text+0x1e02): multiple definition of `set_sense_data'
  drivers/staging/rts_pstor/built-in.o:(.text+0xa79f): first defined here
  drivers/staging/rts5139/built-in.o: In function `ms_delay_write':
  ...

Signed-off-by: Roger Tseng <rogerable@realtek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>