spi: rzv2h-rspi: Fix silent failure in clock setup error path

rzv2h_rspi_setup_clock() is declared to return u32 but returns -EINVAL
when no valid clock parameters are found. Cast to u32, -EINVAL becomes
0xffffffea, which is a non-zero value. The caller in
rzv2h_rspi_prepare_message() guards against failure with:

rspi->freq = rzv2h_rspi_setup_clock(rspi, speed_hz);
if (!rspi->freq)
return -EINVAL;

Because 0xffffffea is non-zero, the check is bypassed and the controller
proceeds to program SPBR/SPCMD with stale values, leading to an unknown
bit rate.

Return 0 on the failed-search path, consistent with the existing
clk_set_rate() failure path which already returns 0.

Fixes: 77d931584dd3 ("spi: rzv2h-rspi: make transfer clock rate finding chip-specific")
Signed-off-by: John Madieu <john.madieu.xa@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Link: https://patch.msgid.link/20260425024725.2393632-1-john.madieu.xa@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>