fix: sync port_fast to APPL_DB on boot when portfast already configured

[salahadell]

Description

After a reboot, stpd initializes g_fastspan_config_mask to all-1s
(PORT_MASK_SET_ALL) as a boot-time default. When stpmgrd replays
persisted CONFIG_DB with portfast=true, stpmgr_config_fastspan()
hits the is_member guard and returns early without calling
stpsync_update_port_fast(). As a result, port_fast is never written
to APPL_DB and show spanning-tree displays PortFast as N even
though it is configured.

This only affects reboots: on first-time setup the CLI creates STP_PORT
entries with portfast=false, which triggers the disable path and clears
the mask bits before the user enables PortFast. After reboot the config
is already true so no disable/clear step occurs, leaving the guard
always firing on the enable path.

Fix

Ensure stpsync_update_port_fast() is called before the early
return in both the enable and disable branches of
stpmgr_config_fastspan(). This guarantees APPL_DB reflects the current
portfast state whenever config is replayed, regardless of the initial
mask value.

Testing

• Configure portfast on an interface, reboot.
• Verify show spanning-tree shows PortFast Y after boot.
• Verify toggling portfast on/off still works correctly at runtime.

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[salahadell]

@divyachandralekha @lihuay , Could you please review this fix? It addresses a boot-time APPL_DB sync issue for PortFast configuration.