refactor(i2s): make the slot_bit_width checks unique

docs/en/api-reference/peripherals/i2s.rst

@@ -855,6 +855,10 @@ Full-duplex mode registers TX and RX channel in an I2S port at the same time, an
 
 Note that one handle can only stand for one channel. Therefore, it is still necessary to configure the slot and clock for both TX and RX channels one by one.
 
+.. note::
+
+    In full-duplex mode, only one channel can work as the master that generates BCLK and WS. If both paired handles are configured as ``I2S_ROLE_MASTER``, the handle initialized later is automatically switched to ``I2S_ROLE_SLAVE``.
+
 There are two methods to allocate a pair of full-duplex channels:
 
 1. Allocate both TX and RX handles in a single call of :cpp:func:`i2s_new_channel`.

docs/zh_CN/api-reference/peripherals/i2s.rst

@@ -849,6 +849,10 @@ STD RX 模式
 
 请注意，一个句柄只能代表一个通道，因此仍然需要对 TX 和 RX 通道逐个进行声道和时钟配置。
 
+.. note::
+
+    全双工模式下只能有一个通道作为 master 生成 BCLK 和 WS。如果配对的两个句柄都配置为 ``I2S_ROLE_MASTER``，后初始化的句柄会被自动切换为 ``I2S_ROLE_SLAVE``。
+
 驱动支持两种分配全双工通道的方法：
 
 1. 在调用 :cpp:func:`i2s_new_channel` 函数时，同时分配 TX 和 RX 通道两个通道。