spi: spi-pl022: Provide missing struct attribute/function param docs

Also demote non-worthy kerneldoc headers to standard comment blocks.

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

 drivers/spi/spi-pl022.c:304: warning: cannot understand function prototype: 'enum ssp_writing '
 drivers/spi/spi-pl022.c:330: warning: Function parameter or member 'loopback' not described in 'vendor_data'
 drivers/spi/spi-pl022.c:398: warning: Function parameter or member 'rx_lev_trig' not described in 'pl022'
 drivers/spi/spi-pl022.c:398: warning: Function parameter or member 'tx_lev_trig' not described in 'pl022'
 drivers/spi/spi-pl022.c:398: warning: Function parameter or member 'dma_running' not described in 'pl022'
 drivers/spi/spi-pl022.c:670: warning: Function parameter or member 'pl022' not described in 'readwriter'
 drivers/spi/spi-pl022.c:1250: warning: Function parameter or member 'irq' not described in 'pl022_interrupt_handler'
 drivers/spi/spi-pl022.c:1250: warning: Function parameter or member 'dev_id' not described in 'pl022_interrupt_handler'
 drivers/spi/spi-pl022.c:1343: warning: Function parameter or member 'pl022' not described in 'set_up_next_transfer'
 drivers/spi/spi-pl022.c:1343: warning: Function parameter or member 'transfer' not described in 'set_up_next_transfer'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Sachin Verma <sachin.verma@st.com>
Link: https://lore.kernel.org/r/20200717135424.2442271-9-lee.jones@linaro.org


Signed-off-by: Mark Brown <broonie@kernel.org>