ext: nffs: portability: Avoid void* arithmetics which is a GNU extension
ext: nffs: portability: Avoid void* arithmetics which is a GNU extension

Under GNU C, sizeof(void) = 1. This commit simply uses uint8_t.

Pointer arithmetics over void types is:
 * A GNU C extension
 * Not supported by Clang
 * Illegal across all ISO C standards

See also: https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html

Signed-off-by: Mark Ruvald Pedersen <3c05dc905675c0722f67ee578faf639727313efa@oticon.com>

galak/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,ldts/zephyr,punitvara/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr