wil6210: guarantee safe access to rx descriptors shared memory

add memory barrier after allocating new rx descriptors, before
updating the hwtail.
This will guarantee that all writes to descriptors (shared memory)
are done before committing them to HW.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>