lspci: Change output for bridge with empty range to "[disabled]"

lspci: Change output for bridge with empty range to "[disabled]"

Change output displayed for memory behind bridge when the range is
empty to be consistent between each verbosity level. Replace "None" and
"[empty]" with "[disabled]". Old and new output examples listed below
for each verbosity level.

Show_range() is not called unless verbose == true. No output given
unless a verbose argument is provided.

OLD output for -v and -vv which uses "None" and -vvv uses "[empty]":

  Memory behind bridge: None                          # lspci -v
  Memory behind bridge: None                          # lspci -vv
  Memory behind bridge: 0000e000-0000efff [empty]     # lspci -vvv

NEW output for -v, -vv, and -vvv to use "[disabled]":

  Memory behind bridge: [disabled]                       # lspci -v
  Memory behind bridge: [disabled]                       # lspci -vv
  Memory behind bridge: 0000e000-0000efff [disabled]     # lspci -vvv

Advantage is consistent output regardless of verbosity level chosen and
to simplify the code.

Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>