drivers: crc: Add 'pad' parameter to crc16()

'pad' parameter controls whether crc16() should add padding at the end
of input bytes or not. This allows to compute CRC16 for data stored in
non-contiguous buffers where CRC value is calculated using subsequent
calls to crc16() with padding added only for last chunk.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>