firmware_loader: Fix a NULL vs IS_ERR() check

The crypto_alloc_shash() function doesn't return NULL, it returns
error pointers.  Update the check accordingly.

Fixes: 02fe26f2 ("firmware_loader: Add debug message with checksum for FW file")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/36ef6042-ce74-4e8e-9e2c-5b5c28940610@kili.mountain


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>