Fix a small syntactic type in tokenizer.c

This change I think is logically equivalent, but certainly "bitwise and"
was not intended here.  gcc 4.8 complains about it:

    astropy/io/ascii/src/tokenizer.c:683:20: warning: suggest parentheses around comparison in operand of ‘&’ [-Wparentheses]
         p += (tsep != '\0' & *p == tsep);