lib-fs: fs_file_init_parent() - Keep mode and flags parameters separated

lib-fs: fs_file_init_parent() - Keep mode and flags parameters separated

Internally cast them both to (int) before ORing them together for the
fs_file_init_with_event() call.

This avoids compiler warnings with -Wenum-enum-conversion:
warning: bitwise operation between different enumeration types ('enum fs_open_mode' and 'enum fs_open_flags')