ARM: SMP: Clean up ncores sanity checks

ARM: SMP: Clean up ncores sanity checks

scu_get_core_count() never returns zero cores, so we don't need to
check and correct if ncores is zero.

Tegra was missing the check against NR_CPUS, leading to a potential
bitfield overflow if this becomes the case.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>