usb: sl811-hcd: only defined function checkdone if QUIRK2 is defined

Function checkdone is only required if QUIRK2 is defined, so add
appropriate #if / #endif around the function.

Cleans up clang scan build warning:
drivers/usb/host/sl811-hcd.c:588:18: warning: unused function
'checkdone' [-Wunused-function]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20240307111351.1982382-1-colin.i.king@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>