[ALSA] usbaudio.c: remove #ifndef CONFIG_USB_EHCI_SPLIT_ISO code

Since USB_EHCI_SPLIT_ISO is now unconditionally enabled the
#ifndef CONFIG_USB_EHCI_SPLIT_ISO became wrong.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>