usb: ohci-omap: avoid unused-variable warning

The dead code removal has led to 'need_transceiver' not being
used at all when OTG support is disabled:

drivers/usb/host/ohci-omap.c: In function 'ohci_omap_reset':
drivers/usb/host/ohci-omap.c:99:33: error: unused variable 'need_transceiver' [-Werror=unused-variable]
   99 |         int                     need_transceiver = (config->otg != 0);

Change the #ifdef check into an IS_ENABLED() check to make the
code more readable and let the compiler see where it is used.

Fixes: 8825acd7 ("ARM: omap1: remove dead code")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>