wil6210: add sysfs for setting connect SNR threshold

snr_thresh sysfs can be used to set omni and direct SNR threshold for
connection.

Change-Id: I091a6b61a1a4cb98e0e8c0f70b2fff4d22486e61
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
CRs-Fixed: 2143032
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>