staging: rts5208: remove unneeded header include path

A header include path without $(srctree)/ is suspicous because it does
not work with O= builds.

I can build drivers/staging/rts5208/ without this include path.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200104163710.21582-1-masahiroy@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>