Fix a small syntactic typo in tokenizer.c

This fix should be logically equivalent, but I don't think "bitwise and"
was really intended here, and gcc 4.8.3 warns about it:

    astropy/io/ascii/src/tokenizer.c:683:20: warning: suggest parentheses around comparison in operand of ‘&’ [-Wparentheses]
         p += (tsep != '\0' & *p == tsep);