i2c: axxia: use 'struct' for kernel-doc notation

Add "struct" to prevent this kernel-doc warning:

drivers/i2c/busses/i2c-axxia.c:135: warning: cannot understand function prototype: 'struct axxia_i2c_dev '
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-axxia.c

@@ -118,7 +118,7 @@
 #define SDA_HOLD_TIME		0x90
 
 /**
- * axxia_i2c_dev - I2C device context
+ * struct axxia_i2c_dev - I2C device context
  * @base: pointer to register struct
  * @msg: pointer to current message
  * @msg_r: pointer to current read message (sequence transfer)