modem: wncm14a2a: fix warning with isalpha/isdigit

When building with arm-clang we get warnings of the type:

wncm14a2a.c:828:14: error: array subscript is of type
			   'char' [-Werror,-Wchar-subscripts]:
                } else if (isalpha(c2) != 0) {
                           ^~~~~~~~~~~

Fix this by adding explicit casts to (int) when callinng isalpha()
or isdigit().

Signed-off-by: Kumar Gala <kumar.gala@intel.com>