staging: rts5139: make the local variable static

Make the local variable (media_not_present and invalid_cmd_field) static

Fix the following sparse warnings in rts51x_transport.c

drivers/staging/rts5139/rts51x_transport.c:649:4: warning: symbol 'media_not_present' was not declared. Should it be static?
drivers/staging/rts5139/rts51x_transport.c:651:4: warning: symbol 'invalid_cmd_field' 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>