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>