staging/rts5208: Fix read overflow in memcpy

Noticed by FORTIFY_SOURCE, this swaps memcpy() for strncpy() to zero-value
fill the end of the buffer instead of over-reading a string from .rodata.

Signed-off-by: Daniel Micay <danielmicay@gmail.com>
[kees: wrote commit log]
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Wayne Porter <wporter82@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>