Staging: rt2860: Avoid extern in .c file

rt28xx_close and rt28xx_open are already declared in a file included in
rt_config.h, so there is no need to declare them again.

Signed-off-by: L. Alberto Giménez <agimenez@sysvalve.es>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>