libata: Change "write_data" to "rw" for some function documentations

The documentation for ata_data_xfer and ata_data_xfer_noirq had the 'rw'
parameter named 'write_data'.

Signed-off-by: Linus Nilsson <lajnold@acc.umu.se>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>