ds2782_battery: Rename get_current to fix build failure / name conflict

This patch changes the name of get_current function pointer to
get_battery_current to resolve a name conflict with the get_current
macro defined in current.h.

This conflict resulted in a build-failure[1] for the sh4 arch
allyesconfig:

  drivers/power/ds2782_battery.c:216:48: error: macro "get_current"
  passed 2 arguments, but takes just

This patch fixes the issue. To be consistent the other function pointers
(_voltage,_capacity) were renamed too.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Acked-by: Ryan Mallon <ryan@bluewatersys.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>