Use os.path.relpath instead of lstrip

Using lstrip sometimes broken the folder names in wifi folder by removing
the "C" in the relpath.

A code similar to the following was added to debug:

if props:
    log.error(f"Unhandled properties found: {props} in file {relpath}")

assert not props

and the result was:

Unhandled properties found: {'': '4355'} in file -4355__s-C1/P-hawaii-ID_M-YSBC_V-m__m-2.3.txt
Traceback (most recent call last):
  File "/home/aditya/firmware.sh", line 905, in <module>
    wifi_col = WiFiFWCollection(sys.argv[1]+"/wifi")
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/aditya/firmware.sh", line 737, in __init__
    self.load(source_path)
  File "/home/aditya/firmware.sh", line 776, in load
    assert not props
AssertionError

Signed-off-by: Aditya Garg <gargaditya08@live.com>

asahi_firmware/wifi.py

@@ -45,7 +45,7 @@ def load(self, source_path):
  dirnames.remove("perf")
  if "assert" in dirnames:
  dirnames.remove("assert")
- subpath = dirpath.lstrip(source_path)
+ subpath = os.path.relpath(dirpath, source_path)
  for name in sorted(filenames):
  if not any(name.endswith("." + i) for i in self.EXTMAP):
  continue