sifive_spi: Add missing case for SPIBUS_MODE_NONE

Otherwise sckmode is left uninitialised, not zero. This mode is used for
the on-board flash on the HiFive Unmatched board. Whilst here, catch
unknown modes and return an error rather than silently continuing.

Reviewed by: riscv, jrtc27
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31562

(cherry picked from commit f5d78bea1f699c05e1694505088e61d22b8fb1f5)