regulator: tps65217: Fix NULL pointer dereference on probe

There is no check that tps->strobes is allocated successfully in
tps65217_regulator_probe().
The patch adds corresponding check.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Signed-off-by: Mark Brown <broonie@kernel.org>