ath79: update Sophos AP15 to indicate that it uses an QCA9557 SoC

ath79: update Sophos AP15 to indicate that it uses an QCA9557 SoC

Device support for Sophos AP15 is based on Sophos AP55(C) and AP100(C).
Those other Sophos access points uss a QCA9558 SoC (some of them with
one of the three chains on the built-in SoC's wifi disabled) while the
AP15 uses a QCA9557 SoC (which only has two chains enabled in the
package or silicon).

This is mostly cosmetic since QCA9558 and QCA9557 are virtually
identical and all differences are automatically detected and/or managed
by the ART calibration.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://github.com/openwrt/openwrt/pull/16187
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>