wimax/i2400m: don't retry SDIO enable in probe() paths

The iwmc3200 has a quirk where retrying SDIO enable during the probe()
path causes bad interactions with the TOP function controller that
causes a reset storm. The workaround is simply not to retry an SDIO
enable in said path (and still do in the reset / reinitialization
paths).

The driver does so by checking i2400ms->debugfs_dentry to see if it
has been initialized; if not, it is in the probe() path. Document said
fact in i2400ms->debugfs_entry.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>