[media] DocBook: fix FE_READ_STATUS argument description

What's written there about the arguments for this ioctl
is bogus: it doesn't return an enum (or a typedef)
for enum fe_status. Instead, it returns a bitmask with the
values defined by enum fe_status.

Also, the size of the integer returned is not 16 bits, but,
instead, sizeof(fe_status_t), e. g. sizeof(enum), with is
arch-dependent.

This should of course be fixed, but this should be done on
a separate patch.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>