wpa_supplicant: Configurable EAP fragment size in AP mode

wpa_supplicant: Configurable EAP fragment size in AP mode

In wpa_supplicant AP mode, allow configuration of the EAP fragment size
using the fragment_size member of network block (wpa_ssid), similar to
the fragment_size in hostapd configuration. bss->fragment_size default
value of 0 is treated specially in some EAP code paths (such as MTU
initialization in eap_pwd_init). In order to preserve the existing
behavior, bss->fragment_size will only be set if the network block
specified a value different from the default which is
DEFAULT_FRAGMENT_SIZE(1398) bytes.

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