Bluetooth: 6lowpan: Rename confusing 'pchan' variables

The typical convention when having both a child and a parent channel
variable is to call the former 'chan' and the latter 'pchan'. When
there's only one variable it's called chan. Rename the 'pchan'
variables in the 6lowpan code to follow this convention.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>