tests/qtest/boot-serial-test: Reduce for() loop in PL011 tests

tests/qtest/boot-serial-test: Reduce for() loop in PL011 tests

Since registers are not modified, we don't need
to refill their values. Directly jump to the previous
store instruction to keep filling the TXDAT register.

The equivalent C code remains:

  while (true) {
      *UART_DATA = 'T';
  }

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>