logging: fix warning of array subscript having type `char`

In the count_s() function, with -Wchar-subscripts, GCC warns
about array subscript having type ‘char’ with the isalpha()
call. Since isalpha() takes an int, so do a type-cast there
to get rid of the warning.

This happens on XCC which is based on GCC 4.2.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>