Re-vist PD minimum buffer size calculation for LibOSDP

[sidcha]

Discussed in #217

^{Originally posted by gandraX January 20, 2025}
Hi,
I implemented libsodp (CP mode) on my device and connected two readers (PD) from different manufacturers. One of them has a Receive Buffer Size of 64B, and CP is unable to send any commands to it after the osdp_CAP command.
It fails in the function osdp_phy_packet_init because of this line:
exp_len = sizeof(struct osdp_packet_header) + 64; /* 64 is estimated */

Thus, libsodp requires the PD to have more than a 64B + header buffer.
I did not find explicit minimum rx buffer requirements for PD in the standard. Are there any?

Thank You