gpio: EN7523: fix kernel-doc warnings

Add "struct" keyword and explain the @dir array differently to
prevent kernel-doc warnings:

gpio-en7523.c:22: warning: cannot understand function prototype: 'struct airoha_gpio_ctrl '
gpio-en7523.c:27: warning: Function parameter or struct member 'dir' not described in 'airoha_gpio_ctrl'
gpio-en7523.c:27: warning: Excess struct member 'dir0' description in 'airoha_gpio_ctrl'
gpio-en7523.c:27: warning: Excess struct member 'dir1' description in 'airoha_gpio_ctrl'

Fixes: 0868ad38 ("gpio: Add support for Airoha EN7523 GPIO controller")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>