Fix CYW43439 receive buffer overflow

[sqt]

Motivation

Control and join polling can receive Ethernet frames larger than their 1,024-byte buffers. Unchecked reads can overwrite the stack and crash firmware during WiFi recovery.

Description of changes

• Size receive buffers for the maximum radio packet length, including alignment and trailing SPI status.
• Reject insufficient buffer capacity before reading SPI.
• Add a mock SPI regression covering oversized-packet rejection and full-frame reception during join polling.