regulator: core: Fix cast to pointer from integer of different size warning

This is to address the following warning during compilation time: (Compile on x86_64)

  CC      drivers/regulator/core.o
drivers/regulator/core.c: In function '_regulator_do_set_voltage':
drivers/regulator/core.c:2183:10: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

This patch adds a temporary variable to avoid double cast.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>