[GCC4] fix warnings in drivers/char/sx.c

[GCC4] fix warnings in drivers/char/sx.c

This patch fixes this warning with gcc 4 :

sx.c: In function 'sx_busy_wait_eq':
sx.c:525: warning: comparisons like X<=Y<=Z do not have their mathematical meaning
sx.c:531: warning: comparisons like X<=Y<=Z do not have their mathematical meaning
sx.c: In function 'sx_busy_wait_neq':
sx.c:551: warning: comparisons like X<=Y<=Z do not have their mathematical meaning
sx.c:557: warning: comparisons like X<=Y<=Z do not have their mathematical meaning