platform/chrome: cros_ec_lpc: Handle zero length read/write

cros_ec_lpc_mec_read_bytes and cros_ec_lpc_mec_write_bytes call
cros_ec_lpc_mec_in_range, which checks if addresses are in the MEC
address range, and returns -EINVAL if the range given is not sensible.

However cros_ec_lpc_mec_in_range was also returning -EINVAL for a zero
length range.

A zero length range should not be an error condition.
cros_ec_lpc_mec_in_range now returns 1 in this case.
cros_ec_lpc_io_bytes_mec checks for zero length, and returns
immediately without beginning a transfer.

Fixes: 68dbac0a58ef ("platform/chrome: cros_ec_lpc: MEC access can return error code")
Fixes: 77a714325d09 ("platform/chrome: cros_ec_lpc: Fix error code in cros_ec_lpc_mec_read_bytes()")
Signed-off-by: Ben Walsh <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Tzung-Bi Shih <[e-mail]>

drivers/platform/chrome/cros_ec_lpc_mec.c

···
  */
 int cros_ec_lpc_mec_in_range(unsigned int offset, unsigned int length)
 {
-	if (length == 0)
-		return -EINVAL;
-
 	if (WARN_ON(mec_emi_base == 0 || mec_emi_end == 0))
 		return -EINVAL;
 
···
 	u8 sum = 0;
 	enum cros_ec_lpc_mec_emi_access_mode access, new_access;
 	int ret;
+
+	if (length == 0)
+		return 0;
 
 	/* Return checksum of 0 if window is not initialized */
 	WARN_ON(mec_emi_base == 0 || mec_emi_end == 0);