iwlwifi: move BT_MBOX_PRINT macro to common header

Move the BT_MBOX_PRINT() macro from mvm/debugfs.c to fw/api/coex.h so
it can be reused and remove duplicate definition of BT_MBOX_MSG(),
keeping only the one already in coex.h.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>