Cisco – A FEX port remains inactive – how can I bring it up

No, it does not. The 2148 FEX has 48 1000Base-T host interfaces. You can confirm this by checking the interface capabilities with the following command (this taken from a 2248 which does support 100Base-TX):

Nexus# show interface ethernet 102/1/3 capabilities
Ethernet102/1/3
  Model:                 N2K-C2248TP-1GE
  Type (Non SFP):        --
  Speed:                 100,1000,auto
  Duplex:                full
  Trunk encap. type:     802.1Q
  Channel:               no
  Broadcast suppression: no
  Flowcontrol:           rx-(off/on),tx-(off/on)
  Rate mode:             none
  QOS scheduling:        rx-(6q1t),tx-(1p6q0t)
  CoS rewrite:           no
  ToS rewrite:           no
  SPAN:                  yes
  UDLD:                  no
  Link Debounce:         yes
  Link Debounce Time:    yes
  MDIX:                  no
  Pvlan Trunk capable:   yes
  TDR capable:           no
  FabricPath capable:    no
  Port mode:             Switched
  FEX Fabric:            no

Whenever you're configuring a FEX switchport, you must ensure that the configuration is consistent across both parent switches (i.e. Nexus 5K). You can do this manually or use the config-sync feature to simplify your configuration.

For instance:

N5K-A(config)# interface e100/1/40
N5K-A(config-int)# switchport mode trunk

HOWEVER - you must repeat this configuration on the same port on the other control plane (other Nexus parent switch) because the configurations are not sync'd automatically by default.

N5K-B(config)# interface e100/1/40
N5K-B(config-int)# switchport mode trunk

If the configuration for a FEX switchport doesn't match on both parent switches, it will place it into "inactive" state as you describe.