wpi(4): check size before transmitting frames

In addition to https://bz-attachments.freebsd.org/attachment.cgi?id=156112;
fixes https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=144987.

Tested:

• Tested with Intel 3945BG, STA mode

Submitted by: <s3erios@gmail.com>
Differential Revision: https://reviews.freebsd.org/D3762