PMIC: MAX77686: fix invalid bus check

Since p->bus is unsigned checking for negative values
is optimized away. Since bus is already used as an argument
use tmp. While at it, don't declare variables in the middle
of a function.

cc: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>