sys: Check return values of chdir and write

sys: Check return values of chdir and write

Fixes the following warnings:

rpcd/sys.c: In function 'rpc_cgi_password_set':
rpcd/sys.c:116:8: error: ignoring return value of 'chdir', declared with attribute warn_unused_result [-Werror=unused-result]
   chdir("/");
        ^
rpcd/sys.c:125:8: error: ignoring return value of 'write', declared with attribute warn_unused_result [-Werror=unused-result]
   write(fds[1], blobmsg_data(tb[RPC_P_PASSWORD]),
        ^
rpcd/sys.c:127:8: error: ignoring return value of 'write', declared with attribute warn_unused_result [-Werror=unused-result]
   write(fds[1], "\n", 1);
        ^
rpcd/sys.c:131:8: error: ignoring return value of 'write', declared with attribute warn_unused_result [-Werror=unused-result]
   write(fds[1], blobmsg_data(tb[RPC_P_PASSWORD]),
        ^
rpcd/sys.c:133:8: error: ignoring return value of 'write', declared with attribute warn_unused_result [-Werror=unused-result]
   write(fds[1], "\n", 1);
        ^
cc1: all warnings being treated as errors

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>