i2c: axxia: eliminate kernel-doc warnings

Add kernel-doc for 'slave' and 'irq' in struct axxia_i2c_dev.
Drop kernel-doc notation ("/**") for static functions since they
are not usually documented with kernel-doc.

Prevents these kernel-doc warnings:

i2c-axxia.c:150: warning: Function parameter or member 'slave' not described in 'axxia_i2c_dev'
i2c-axxia.c:150: warning: Function parameter or member 'irq' not described in 'axxia_i2c_dev'
i2c-axxia.c:172: warning: Function parameter or member 'ns' not described in 'ns_to_clk'
i2c-axxia.c:172: warning: Function parameter or member 'clk_mhz' not described in 'ns_to_clk'
i2c-axxia.c:172: warning: No description found for return value of 'ns_to_clk'
i2c-axxia.c:271: warning: Function parameter or member 'idev' not described in 'axxia_i2c_empty_rx_fifo'
i2c-axxia.c:271: warning: No description found for return value of 'axxia_i2c_empty_rx_fifo'
i2c-axxia.c:303: warning: Function parameter or member 'idev' not described in 'axxia_i2c_fill_tx_fifo'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/all/202310181049.Vo62moV1-lkp@intel.com/


Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>