Bluetooth: Kconfig: Fix error in buffer pool sizes

Fix error in calculation of the minimum discardable buffer size. For
the LE Advertising Report the maximum payload is 31 bytes plus
additional data in the event gives an event size of 41 bytes (given that
num_reports is 1). Since this is a meta event we need to include the
sub-event code, plus the event header of 2 bytes. Total of 44 bytes.

This is a regression from afa9c42d where we forgot th 1 byte for the
RSSI that is appended after the data. Easy to miss since it is not part
of the struct.

Fix error in calculation of the num complete buffer size. Here we forgot
to include the 2 byte event header.

This is a regression from 89981b07.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>