spi: use of_property_read_bool()

Use a bit more compact of_property_read_bool() calls instead of the
of_find_property() calls -- symmetrically with the of_property_read_u32()
calls already done in of_spi_parse_dt().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Mark Brown <broonie@kernel.org>