isofs: Use *-y instead of *-objs in Makefile

*-objs suffix is reserved rather for (user-space) host programs while
usually *-y suffix is used for kernel drivers (although *-objs works
for that purpose for now).

Let's correct the old usages of *-objs in Makefiles.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240508152129.1445372-1-andriy.shevchenko@linux.intel.com>