spi: bitbang: Fix lsb-first Rx

Shifting the recieved bit by "bits" inserts it at the top of the
*currently remaining* Tx data, so we end up accumulating the whole
transfer into bit 0 of the output word. Oops.

For the algorithm to work as intended, we need to remember where the
top of the *original* word was, and shift Rx to there.

Fixes: 1847e3046c52 ("spi: gpio: Implement LSB First bitbang support")
Signed-off-by: Robin Murphy <[e-mail]>
Link: https://lore.kernel.org/r/2832[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-bitbang-txrx.h

···
 {
 	/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
 
+	u8 rxbit = bits - 1;
 	u32 oldbit = !(word & 1);
 	/* clock starts at inactive polarity */
 	for (; likely(bits); bits--) {
···
 		/* sample LSB (from slave) on leading edge */
 		word >>= 1;
 		if ((flags & SPI_MASTER_NO_RX) == 0)
-			word |= getmiso(spi) << (bits - 1);
+			word |= getmiso(spi) << rxbit;
 		setsck(spi, cpol);
 	}
 	return word;
···
 {
 	/* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
 
+	u8 rxbit = bits - 1;
 	u32 oldbit = !(word & 1);
 	/* clock starts at inactive polarity */
 	for (; likely(bits); bits--) {
···
 		/* sample LSB (from slave) on trailing edge */
 		word >>= 1;
 		if ((flags & SPI_MASTER_NO_RX) == 0)
-			word |= getmiso(spi) << (bits - 1);
+			word |= getmiso(spi) << rxbit;
 	}
 	return word;
 }