wifi: rtlwifi: Speed up firmware loading for USB

Currently it takes almost 6 seconds to upload the firmware for RTL8192CU
(and 11 seconds for RTL8192DU). That's because the firmware is uploaded
one byte at a time.

Also, after plugging the device, the firmware gets uploaded three times
before a connection to the AP is established.

Maybe this is fine for most users, but when testing changes to the
driver it's really annoying to wait so long.

Speed up the firmware upload by writing chunks of 64 bytes at a time.
This way it takes about 110 ms for RTL8192CU (and about 210 ms for
RTL8192DU).

PCI devices could upload it in chunks of 4 bytes, but I don't have any
to test and commit 89d32c90 ("rtlwifi: Download firmware as bytes
rather than as dwords") decided otherwise anyway.

Allocate memory for the firmware image with kmalloc instead of vzalloc
because this memory is passed directly to usb_control_msg(), which
can't take memory allocated by vmalloc.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/d9bd4949-6e92-4f35-8b60-3b45f9ad74ab@gmail.com