tprintf: Fix compiler warnings

Fix the following warning when compiling with
-W/-Wsign-compare/-Wunused-parameter:

tprintf.c: In function ‘__tprintf_flush_skip’:
tprintf.c:54:65: warning: unused parameter ‘max’ [-Wunused-parameter]
tprintf.c: In function ‘__tprintf_flush’:
tprintf.c:70:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
tprintf.c:76:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
tprintf.c:80:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
tprintf.c: In function ‘tprintf’:
tprintf.c:131:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>