lib/posix/fs: fix end-of-dir in readdir
lib/posix/fs: fix end-of-dir in readdir

POSIX readdir should return NULL if end of dir is reached and
leave errno untouched.

Signed-off-by: Rico Ganahl <9a8c49ebbd2d5b39f9f1ec3c824fa965f9547452@bytesatwork.ch>

finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr