i2c: busses: Use fallthrough pseudo-keyword

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-through



Acked-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>