staging: rtl8712: Use linux/atomic.h instead of asm/atomic.h

Fixes warning from scripts/checkpatch.pl.

Signed-off-by: Michael Fiedler <michael.fiedler@informatik.stud.uni-erlangen.de>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>