spi: spi-rpc-if: Check return value of rpcif_sw_init()

rpcif_sw_init() can fail so make sure we check the return value
of it and on error exit rpcif_spi_probe() callback with error code.

Fixes: eb8d6d46

 ("spi: add Renesas RPC-IF driver")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211025205631.21151-4-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>