common/vsprintf: fix signed->unsigned error, causing glacial performance

common/vsprintf: fix signed->unsigned error, causing glacial performance

The original patch for

  c/s 67a3542c5bc356e6452d8305991617c875f87de4
  "common/vsprintf: Refactor string() out of vsnprintf()"

specifically used signed integers, identical to the code copied out of vsprintf.

When committed, these had changed to unsigned integers, which causes a
functional change.  This causes glacial boot performance and an excessive
quantity of spaces printed to the serial console, as we loop to the upper
bound of a 32bit integer.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>