regulator: axp20x: remove a redundant null check on rdev

Currently rdev is dereferenced when assigning desc before rdev is null
checked, this is leading to static analysis warnings.  However, rdev
can never be null, so the null check is redundant and can be removed.

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>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>