LOCAL / net: bcm4358: call vfs functions instead of direct file ops

LOCAL / net: bcm4358: call vfs functions instead of direct file ops

After enable writing the card's MAC address to filesystem by:

commit: LOCAL / net: bcm4358: Enable the some configurations

in the driver's code for bcmdhd4358, filesystem access is required
to read/write some settings, especially the card's MAC address.

Instead of calling proper fs functions, a direct calls were used
to particular file operations methods.
Since read/write operations are deprecated, the present code without
null pointer checking, refers to null function pointers.
This causes kernel panic when runs wpa_supplicant application.

This commit moves the direct calls to proper function calls,
which fixes the panic issue.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>