bitfield-c: use unsigned int instead of uint8_t

bitfield-c: use unsigned int instead of uint8_t

Use unsigned int instead of uint8_t upon destination and source array length.
This is needed to handle gathered multi-frames message data which could
be greater than 1 simple messages.

Change-Id: I107bff383c2d0771dbc2a30770ec5c195b1c22ac
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>