pinctrl: single: 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/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200716212317.GA17754@embeddedorSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-single.c

@@ -563,7 +563,7 @@ static int pcs_pinconf_set(struct pinctrl_dev *pctldev,
 			case PIN_CONFIG_BIAS_PULL_UP:
 				if (arg)
 					pcs_pinconf_clear_bias(pctldev, pin);
-				/* fall through */
+				fallthrough;
 			case PIN_CONFIG_INPUT_SCHMITT_ENABLE:
 				data &= ~func->conf[i].mask;
 				if (arg)