selftests/powerpc/dscr: Restore timeout to DSCR selftests

Reducing the time taken by dscr_sysfs_test.c allows restoring the
default timeout, which was removed in
commit 850507f3 ("selftests/powerpc: Turn off timeout setting for
benchmarks, dscr, signal, tm") because that test took too long.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230406043320.125138-8-bgray@linux.ibm.com