wlcore: refactor threaded IRQ routine

Separate the threaded IRQ handling routine into two functions.
The outer function takes the mutex and calls recovery on errors. It also
performs a Tx-path optimization to avoid redundant works.

The inner function is simplified - all calls to recovery are removed and
it assumes the lock is taken. The locked variant will be reused elsewhere.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>