V4L/DVB (6142): cx25840: fix build warning

Fix the following build warning:

 CC [M]  cx25840-core.o

cx25840-core.c: In function 'init_dll1':
cx25840-core.c:147: warning: implicit declaration of function 'udelay'


Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>