spi/sh-hspi: fix return value check in hspi_probe().

According to its documentation, clk_get() returns a "valid IS_ERR() condition
containing errno", so we should call IS_ERR() rather than a NULL check.

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