i2c: altera: Fix formatting issue in struct and demote unworthy kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/i2c/busses/i2c-altera.c:74: warning: cannot understand function prototype: 'struct altr_i2c_dev '
 drivers/i2c/busses/i2c-altera.c:180: warning: Function parameter or member 'idev' not described in 'altr_i2c_transfer'
 drivers/i2c/busses/i2c-altera.c:180: warning: Function parameter or member 'data' not described in 'altr_i2c_transfer'
 drivers/i2c/busses/i2c-altera.c:193: warning: Function parameter or member 'idev' not described in 'altr_i2c_empty_rx_fifo'
 drivers/i2c/busses/i2c-altera.c:209: warning: Function parameter or member 'idev' not described in 'altr_i2c_fill_tx_fifo'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>