spi: hisi-sfc-v3xx: factor out bus config and transfer functions

In hisi_sfc_v3xx_generic_exec_op(), we will write the data to the buffer,
configure and start the transfer, read the data to the buffer and check
whether occurs an error. Factor out the config and transfer start codes
as individual functions, to make the process a bit clearer.

Acked-by: John Garry <john.garry@huawei.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/1600950270-52536-3-git-send-email-yangyicong@hisilicon.com


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