isdn: fix obvious cut-and-paste error in st5481_usb.c

Fix a rather obvious cut-and-paste error, where earlier code for the
controller URB got somehow mixed in with code for the interrupt URB.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>