dw-xdata: Use str_write_read() in dw_xdata_start() and dw_xdata_perf()

dw-xdata: Use str_write_read() in dw_xdata_start() and dw_xdata_perf()

Remove hard-coded strings by using the str_write_read() helper function
and silence the following two Coccinelle/coccicheck warnings reported by
string_choices.cocci:

  opportunity for str_write_read(write)
  opportunity for str_write_read(write)

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20250820102108.760382-2-thorsten.blum@linux.dev
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>