Bluetooth: fix sco_exit compile warning

While compiling the following warning occurs:

WARNING: net/built-in.o(.init.text+0x602c): Section mismatch in
reference from the function bt_init() to the function
.exit.text:sco_exit()
The function __init bt_init() references
a function __exit sco_exit().
This is often seen when error handling in the init function
uses functionality in the exit path.
The fix is often to remove the __exit annotation of
sco_exit() so it may be used outside an exit section.

Since commit 6d785aa3

 ("Bluetooth:
Convert mgmt to use HCI chan registration API") the function "sco_exit"
is used inside of function "bt_init". The suggested solution by remove
the __exit annotation solved this issue.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>