tools: iio: iio_generic_buffer: Fix read size

When noevents is true and small buffer is used the allocated memory for
holding the data may be smaller than the hard-coded 64 bytes. This can
cause the iio_generic_buffer to crash.

Following was recorded on beagle bone black with v6.0 kernel and the
digit fix patch:
https://lore.kernel.org/all/Y0f+tKCz+ZAIoroQ@dc75zzyyyyyyyyyyyyycy-3.rev.dnainternet.fi/
using valgrind;

==339== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==339== Command: /iio_generic_buffer -n kx022-accel -T0 -e -l 10 -a -w 2000000
==339== Parent PID: 307
==339==
==339== Syscall param read(buf) points to unaddressable byte(s)
==339==    at 0x496BFA4: read (read.c:26)
==339==    by 0x11699: main (iio_generic_buffer.c:724)
==339==  Address 0x4ab3518 is 0 bytes after a block of size 160 alloc'd
==339==    at 0x4864B70: malloc (vg_replace_malloc.c:381)
==339==    by 0x115BB: main (iio_generic_buffer.c:677)

Fix this by always using the same size for reading as was used for
data storage allocation.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/Y0kMh0t5qUXJw3nQ@dc75zzyyyyyyyyyyyyycy-3.rev.dnainternet.fi
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>