wl3501_cs: Remove unnecessary NULL check

In wl3501_detach(), link->priv is checked for a NULL value before being
passed to free_netdev(). However, it cannot be NULL at this point as it
has already been passed to other functions, so just remove the check.

Addresses-Coverity: CID 710499: Null pointer dereferences (REVERSE_INULL)
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200926174558.9436-1-alex.dewar90@gmail.com