lib: change default number of transfers to 15

So far we had 32 * 256KB which was a bit overkill,
15 are more than enough.

15 was chosen instead of 16 because at least on Linux
there seems to be a system-wide limit of 63 transfers
(when they are 256KB large), so 4 dongles can be used
on a single machine without lowering the default transfer
number.

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_tcp: make all global variables static

Signed-off-by: Steve Markgraf <steve@steve-m.de>