rtl8723au: Fix build problem with dkms

It has been reported that under Ubuntu 13.10 (at least), the 8723au module
won't auto-build correctly with new kernels. The error can be seen explicitly
with dkms status, which gives:
dkms.conf: Error! No 'BUILT_MODULE_NAME' directive

Making BUILT_MODULE_NAME the same as BUILD_MODULE_NAME fixes the problem.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>