staging: rts5139: device_reset() can be static

Function device_reset() is only used in rts51x_scsi.c, so make it be
static.  Otherwise, it will have a name collision with the new added
reset API.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>