rndis_wlan: Remove logically dead code

caps_buf is always of size sizeof(*caps) because
sizeof(caps->auth_encr_pair) * 16 is always zero. Notice
that when using zero-length arrays, sizeof evaluates to zero[1].

So, the code introduced by
commit 0308383f

 ("rndis_wlan: get max_num_pmkids from device")
is logically dead, hence is never executed and can be removed. As a
consequence, the rest of the related code can be refactored a bit.

Notice that this code has been out there since March 2010.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200505235205.GA18539@embeddedor
Link: https://lore.kernel.org/r/20200507110741.37757-1-yanaijie@huawei.com