wifi: rt2x00: correct MAC_SYS_CTRL register RX mask in R-Calibration

For MAC_SYS_CTRL register, Bit[2] controls MAC_TX_EN and Bit[3]
controls MAC_RX_EN (Bit index starts from 0). Therefore, 0x08 is
the correct mask for RX.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/TYAP286MB03150B571B67B896A504AC34BC0EA@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM