mfd: cros_ec_spi: Repair comparison ordering issue

WARNING: Comparisons should place the constant on the right side of the test
+       BUG_ON(EC_MSG_PREAMBLE_COUNT > ec_dev->din_size);

WARNING: Comparisons should place the constant on the right side of the test
+       BUG_ON(EC_MSG_PREAMBLE_COUNT > ec_dev->din_size);

total: 0 errors, 2 warnings, 731 lines checked

Signed-off-by: Lee Jones <lee.jones@linaro.org>