test/py: spi: Set the expected error message

If erase/write/read size is 0 then it throws the mentioned error message
when debug message ie enabled as per 899fb5aa8bec ("cmd: sf/nand: Print
and return failure when 0 length is passed"), setting it to None as
debug message is not enabled by default for testing.

Signed-off-by: Love Kumar <love.kumar@amd.com>
diff --git a/test/py/tests/test_spi.py b/test/py/tests/test_spi.py
index c81eca5..3160d58 100644
--- a/test/py/tests/test_spi.py
+++ b/test/py/tests/test_spi.py
@@ -646,7 +646,7 @@
 
         # If erase size is 0
         esize = 0
-        error_msg = 'ERROR: Invalid size 0'
+        error_msg = None
         flash_ops(
             u_boot_console, 'erase', start, esize, 0, 1, error_msg, EXPECTED_ERASE
         )
@@ -685,7 +685,7 @@
         # if Write/Read size is 0
         offset = random.randint(0, 2)
         size = 0
-        error_msg = 'ERROR: Invalid size 0'
+        error_msg = None
         flash_ops(
             u_boot_console, 'write', offset, size, addr, 1, error_msg, EXPECTED_WRITE
         )