flash/nor/nrf5: pass unsigned char to isalnum()

flash/nor/nrf5: pass unsigned char to isalnum()

In newlib, the argument of isalnum() and the similar functions in
ctype.h is checked to be either an int or an unsigned char.
Using a normal (signed) char triggers a compile time warning
	warning: array subscript has type ‘char’ [-Wchar-subscripts]

Rewrite the function to separate the internal unsigned char
operations from the (signed) char parameter.

Change-Id: I5f19115f0b2de2b5b35dc07ef4b58a96161268ee
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Åke Rehnman <ake.rehnman@gmail.com>
Fixes: 5da746fa09 ("flash/nor/nrf5: detect newer devices without HWID table")
Reviewed-on: http://openocd.zylin.com/5545
Tested-by: jenkins
Reviewed-by: Ake Rehnman <ake.rehnman@gmail.com>