staging: rts5139: make thrts5139_usb_idse local variable static in rts51x.c

Make the local variable rts5139_usb_ids static

Fix the following sparse warnings in rts51x.c

CHECK   drivers/staging/rts5139/rts51x.c
drivers/staging/rts5139/rts51x.c:835:22: warning: symbol 'rts5139_usb_ids' was not declared. Should it be static?

Signed-off-by: Ana Rey <anarey@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>