wcnss: fix the buffer overflow in MAC address store sysfs

Invalid typecast in MAC address store sysfs entry method
causing device crash due to the buffer overflow.

To fix the above issue check the length of the userspace MAC
address before write. Update the datatype and parsing method
to store the userspace MAC address.

CRs-Fixed: 2034549
Change-Id: I0d4709c5b623c8333a99991c042552df1e7da923
Signed-off-by: Sarada Prasanna Garnayak <sgarna@codeaurora.org>