regulator: axp20x: check rdev is null before dereferencing it

Currently rdev is dereferenced when assigning desc before rdev is null
checked, hence there is a potential null pointer dereference on rdev.
Fix this by null checking rdev first.

Detected by CoverityScan, CID#1476031 ("Dereference before null check")

Fixes: 77e3e3b1 ("regulator: axp20x: add software based soft_start for AXP209 LDO3")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>