Use `git am` instead of `b4 shazam`

There appears to be problems with `b4 am` with binary diffs, but there
also doesn't seem to be need to use b4 in this case.

Switch over to just applying the mbox directly.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

contrib/process_linux_firmware.py

@@ -168,7 +168,7 @@ def process_patch(mbox, num, remote):
     quiet_cmd(cmd)
 
     # apply the patch
-    cmd = ["b4", "shazam", "-m", "-"]
+    cmd = ["git", "am"]
     logging.debug("Running {}".format(cmd))
     p = subprocess.Popen(
         cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE