spi: microchip-core: fix passing zero to PTR_ERR warning

spi: microchip-core: fix passing zero to PTR_ERR warning

It is possible that the error case for devm_clk_get() returns NULL,
in which case zero will be passed to PTR_ERR() as shown by the Smatch
static checker warning:
drivers/spi/spi-microchip-core.c:557 mchp_corespi_probe()
warn: passing zero to 'PTR_ERR'

Remove the NULL check and carry on with a dummy clock in case of an
error. To avoid a potential div zero, abort calculating clkgen if
clk_get_rate(spi->clk) is zero.

Fixes: 9ac8d17694b6 ("spi: add support for microchip fpga spi controllers")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/linux-spi/20220615091633.GI2168@kadam/
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220615142028.2991915-1-conor.dooley@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>