drivers: can: mcp25xxfd: Remove return 0 for void function

mcp25xxfd.c:4151:2: error: void function 'mcp25xxfd_debugfs_add'
should not return a value [-Wreturn-type].

Change-Id: I00699349a5317e3b584afd78dd3d8493adf5191d
Signed-off-by: Zhenlin Lian <lianzhenlin@codeaurora.org>