wifi: rtl8xxxu: retry firmware download on error

Occasionally there is an EPROTO error during firmware download.
This error is converted to EAGAIN in the download function.
But nobody tries again and so device probe fails.

Implement download retry to fix this.

This error was observed (and fix tested) on a tbs2910 board [1]
with an embedded RTL8188EU (0bda:8179) device behind a USB hub.

[1] arch/arm/boot/dts/nxp/imx/imx6q-tbs2910.dts

Signed-off-by: Soeren Moch <smoch@web.de>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250127194828.599379-1-smoch@web.de