lib: Improve _parse_integer_fixup_radix base 16 detection

Base autodetection is failing for this case:
if test 257 -gt 3ae; then echo first; else echo second; fi

It is because base for 3ae is recognized by _parse_integer_fixup_radix() as
10. The code detects the first char which is not between 'a'/'A' or 'f'/'F'
to change base from dec to hex.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Shiril Tichkule <shirilt@xlinx.com>