console: file should always be non-negative

We use the parameter file in console functions to choose from an array
after checking against MAX_FILES but we never check if the value of file
is negative.

Running ./u-boot -T -l and issuing the poweroff command has resulted in
crashes because os_exit() results in std::ostream::flush() calling U-Boot's
fflush with file being a pointer which when converted to int may be
represented by a negative number.

This shows that checking against MAX_FILES is not enough. We have to ensure
that the file argument is always positive.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>