raop: Fix gcc-7 warnings, EWOULDBLOCK

raop: Fix gcc-7 warnings, EWOULDBLOCK

EAGAIN is used allover the code rather than EWOULDBLOCK
POSIX allows EAGAIN and EWOULDBLOCK to have the same value (and in fact it is)
don't check for EWOULDBLOCK

modules/raop/raop-client.c: In function ‘send_udp_audio_packet’:
modules/raop/raop-client.c:473:41: warning: logical ‘or’ of equal expressions [-Wlogical-op]
     if (written < 0 && (errno == EAGAIN || errno == EWOULDBLOCK)) {
                                         ^~
modules/raop/raop-client.c: In function ‘resend_udp_audio_packets’:
modules/raop/raop-client.c:528:45: warning: logical ‘or’ of equal expressions [-Wlogical-op]
         if (written < 0 && (errno == EAGAIN || errno == EWOULDBLOCK)) {
                                             ^~

Change-Id: Ifbad3e3cefdf66e59053853894ff51e74d19a3a4
Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>