sms: Fix alphanumeric TP-OA handling

sms: Fix alphanumeric TP-OA handling

TP-OA max length comparisons were incorrect because TP-OA's 7-bit
coded octets transport eleven 8-bit chars.  The current code assumed
only 10 chars were possible.

The patch
- increases the array size to 23, (maximum of 22 bytes for UTF8
  encoding + null terminator)
- Updates the sanity check to account for the correct maximum
- For encoding, checks the maximum length in UTF8 characters instead of
  bytes