NFC: trf7970a: Ignore antenna noise when checking for RF field

NFC: trf7970a: Ignore antenna noise when checking for RF field

The main channel Received Signal Strength Indicator (RSSI) measurement
is used to determine whether an RF field is present or not. RSSI != 0
is interpreted as an RF Field is present. This does not take RF noise
and measurement inaccuracy into account, and results in false positives
in the field.

Define a noise level and make sure the RF field is only interpreted as
present when the RSSI is above the noise level.

Fixes: 851ee3cbf850 ("NFC: trf7970a: Don't turn on RF if there is already an RF field")
Signed-off-by: Paul Geurts <paul.geurts@prodrive-technologies.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Link: https://patch.msgid.link/20260422100930.581237-1-paul.geurts@prodrive-technologies.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>