[SX127x] Fix missing CRC mismatch error

[GUVWAF]

After upgrading to version 7.0 we saw that SX127x radios sometimes got garbled data through. Seems due to #1184 the variable state gets overwritten (normally to 0) and thus it would loose a potential RADIOLIB_ERR_CRC_MISMATCH set before this.
I therefore introduced a new variable for it, and I also moved the SPI calls outside the else clause, because the data might still be valuable even if the header is damaged.

[jp-bennett]

Just tested this with Meshtastic master and a T-Beam with the older RF95 chipset. With this patch, we are once again catching CRC errors. Thanks!

[jgromes]

Good catch - merged, thank you! This looks like a rather serious issue (price we pay for not releasing more often I suppose ...), I will see if we can get 7.0.1 out in the next few days.