net: ethtool: Fix documentation of ethtool_sprintf()

This function takes a pointer to a pointer, unlike sprintf() which is
passed a plain pointer. Fix up the documentation to make this clear.

Fixes: 7888fe53 ("ethtool: Add common function for filling out strings")
Cc: Alexander Duyck <alexanderduyck@fb.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20231028192511.100001-1-andrew@lunn.ch


Signed-off-by: Paolo Abeni <pabeni@redhat.com>