lib: os: hex: correct explicit cast type

change explicit type cast of essential character type, complying with
required [misra-c2012-10.2] rule which states; Expressions of
essentially character type shall not be used inappropriately in addition
and subtraction operations.

Found as a coding guideline violation (Rule 10.2) by static code
scanning tool.

Note: Tested on STM32L5 Nucleo-144 board (stm32l552xx).

Signed-off-by: ferar alashkar <ferar.alashkar@gmail.com>