Bluetooth: Kconfig: Merge headroom reserve variables into a single one

A subsequent patch will start reusing HCI command buffers for
receiving the response, so the distinction of received vs sent data
headroom would just make the code unnecessarily complex. Instead, just
merge these two variable into a single one.

Change-Id: I31d846331939f1a2270df7ed0c75112825e16493
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>