Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughAcked-by: NBaruch Siach <baruch@tkos.co.il>
Reviewed-by: NJean Delvare <jdelvare@suse.de>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Reviewed-by: NGregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NWolfram Sang <wsa@kernel.org>