staging: r8188eu: add firmware dependency

The old rtl8188eu module, removed in commit 55dfa29b

 ("staging:
rtl8188eu: remove rtl8188eu driver from staging dir") (Linux kernel
v5.15-rc1), required (through a MODULE_FIRMWARE call()) the
rtlwifi/rtl8188eufw.bin firmware file, which the new r8188eu driver no
longer requires.

I have tested a few RTL8188EUS-based Wi-Fi cards and, while supported by
both drivers, they do not work when using the new one and the firmware
wasn't manually loaded. According to Larry Finger, the module
maintainer, all such cards need the firmware and the driver should
depend on it (see the linked mails).

Add a proper MODULE_FIRMWARE() call, like it was done in the old driver.

Thanks to Greg Kroah-Hartman and Larry Finger for quick responses to my
questions.

Cc: stable <stable@kernel.org>
Link: https://answers.launchpad.net/ubuntu/+source/linux-meta-hwe-5.15/+question/702611
Link: https://lore.kernel.org/lkml/YukkBu3TNODO3or9@nx64de-df6d00/
Signed-off-by: Grzegorz Szymaszek <gszymaszek@short.pl>
Link: https://lore.kernel.org/r/YulcdKfhA8dPQ78s@nx64de-df6d00
Acked-by: Phillip Potter <phil@philpotter.co.uk>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>