fs: zms: make zms_active_sector_free_space return type signed

zms_active_sector_free_space is documented to return -EACCES on fs not
mounted error but currently returns an usigned type size_t. This fixes
that by changing the return value into a ssize_t

Signed-off-by: Theis Mejnertsen <theismejnertsen@gmail.com>