Before splitting the read/write routines into smaller, more
specialized functions, unduplicate some code in advance.

Use a 'for' loop instead of 'do while' when waiting for the previous
write to complete and hide it behind a macro.
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>