nvmem: core: remove unused nvmem_device ncells member

nvmem ncells can be over written by calling nvmem_add_cells()
multiple times. I see there is no real point of maintaining count
of cells when we have a list of cell.

Remove this to avoid any confusion!

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>