rtl8xxxu: Remove set but not used variable 'vif','dev','len'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c: In function rtl8xxxu_c2hcmd_callback:
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:5396:24: warning: variable vif set but not used [-Wunused-but-set-variable]
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c: In function rtl8xxxu_c2hcmd_callback:
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:5397:17: warning: variable dev set but not used [-Wunused-but-set-variable]
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c: In function rtl8xxxu_c2hcmd_callback:
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:5400:6: warning: variable len set but not used [-Wunused-but-set-variable]

They are introduced by commit e542e66b ("rtl8xxxu:
add bluetooth co-existence support for single antenna"), but never used,
so remove them.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>