spi: sh-hspi: control spi clock more correctly

Current sh-hspi had used platform-specific speed.
This patch remove it, and use spi_transfer specific speed.
It removes unnecessary flags from struct sh_hspi_info,
but struct sh_hspi_info is still exist, since sh-hspi needs
platform info in the future.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>