P2P: Adjust service discovery maximum fragment size for 60 GHz

P2P: Adjust service discovery maximum fragment size for 60 GHz

In the 60 GHz band, service discovery management frames are sent over
the control PHY and have a smaller maximum frame size (IEEE Std
802.11ad-2012, 21.4.3.2). Fix the code to use sufficiently small
fragment size when operating in the 60 GHz band.

The 60 GHz fragment size (928) is derived from the maximum frame size
for control PHY (1023) and subtracting 48 bytes of header size, and some
spare so we do not reach frames with the absolute maximum size.

Signed-off-by: Lior David <qca_liord@qca.qualcomm.com>