wifi: rtl8xxxu: remove rtl8xxxu_ prefix from filenames

The driver is already in a directory named rtl8xxxu, there's no need to
duplicate that in the filename as well. Now file listing looks a lot more
reasonable:

8188e.c  8192c.c  8192f.c  8723a.c  core.c   Makefile  rtl8xxxu.h
8188f.c  8192e.c  8710b.c  8723b.c  Kconfig  regs.h

No functional changes, compile tested only.

Signed-off-by: Kalle Valo <kvalo@kernel.org>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/20240426141939.3881678-2-kvalo@kernel.org