block: constify sect_buf argument of ide_write_data

Add a const keyword to the sect_buf argument of
ide_write_data to fix the following warning:

  cmd_ide.c: In function '__ide_output_data':
  cmd_ide.c:548: warning: passing argument 2 of 'ide_write_data' discards qualifiers from pointer target type
  /devel/u-boot.git/include/ide.h:76: note: expected 'ulong *' but argument is of type 'const ulong *'

Also modify the driver-model documentation to
match with the new prototype.

Compile tested only.

Cc: Macpaul Lin <macpaul@andestech.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>