chan_sip: in case of tcp/tls, be less annoying about tx errors.

chan_sip:  in case of tcp/tls, be less annoying about tx errors.

chan_sip.c:3782 __sip_xmit: sip_xmit of 0x7f1478069230 (len 600) to
213.150.203.60:1492 returned -2: Interrupted system call

returned -2 implies this wasn't actually an OS error, so errno makes no
sense either.  Internal error was already logged higher up, and -2
generally means that either there isn't a valid connection available, or
the pipe notification failed, and that is already correctly logged.

ASTERISK-28651 #close

Change-Id: I46eb82924beeff9dfd86fa6c7eb87d2651b950f2
Signed-off-by: Jaco Kroon <jaco@uls.co.za>