drivers: spi_sifive: Use uint8_t data/buffer types to match spi_context.

spi_context uses uint8_t buffers, and HW rxdata / txdata registers only
contain 8 bits of data (along with b31 full / empty flag), so uint8_t is
appropriate.

Suggested-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
Signed-off-by: Shawn Nematbakhsh <shawn@rivosinc.com>