spi: atmel: simplify MR register update in cs_activate()

simplified the MR register configuration by updating only the PCS field
using SPI_BFINS() instead of rewriting the entire register.
Avoids code duplication.

Signed-off-by: Manikandan Muralidharan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-atmel.c

···
 		 * on CS1,2,3 needs SPI_CSR0.BITS config as SPI_CSR1,2,3.BITS
 		 */
 		spi_writel(as, CSR0, asd->csr);
-		if (as->caps.has_wdrbt) {
-			spi_writel(as, MR,
-					SPI_BF(PCS, ~(0x01 << chip_select))
-					| SPI_BIT(WDRBT)
-					| SPI_BIT(MODFDIS)
-					| SPI_BIT(MSTR));
-		} else {
-			spi_writel(as, MR,
-					SPI_BF(PCS, ~(0x01 << chip_select))
-					| SPI_BIT(MODFDIS)
-					| SPI_BIT(MSTR));
-		}
 
 		mr = spi_readl(as, MR);
+		mr = SPI_BFINS(PCS, ~(0x01 << chip_select), mr);
+		spi_writel(as, MR, mr);
 
 		/*
 		 * Ensures the clock polarity is valid before we actually