RISC-V: simplify register width check in ISA string parsing

Saving off the `isa` pointer to a temp variable, followed by checking if
it has been incremented is a bit of an odd pattern. Perhaps it was done
to avoid a funky looking if statement mixed with the ifdeffery.

Now that we use IS_ENABLED() here just return from the parser as soon as
we detect a mismatch between the string and the currently running
kernel.
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Link: https://lore.kernel.org/r/20230607-splatter-bacterium-a75bb9f0d0b7@spudSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/cpufeature.c

@@ -126,7 +126,6 @@ void __init riscv_fill_hwcap(void)
 	for_each_possible_cpu(cpu) {
 		unsigned long this_hwcap = 0;
 		DECLARE_BITMAP(this_isa, RISCV_ISA_EXT_MAX);
-		const char *temp;
 
 		if (acpi_disabled) {
 			node = of_cpu_device_node_get(cpu);
@@ -149,14 +148,14 @@ void __init riscv_fill_hwcap(void)
 			}
 		}
 
-		temp = isa;
-		if (IS_ENABLED(CONFIG_32BIT) && !strncasecmp(isa, "rv32", 4))
-			isa += 4;
-		else if (IS_ENABLED(CONFIG_64BIT) && !strncasecmp(isa, "rv64", 4))
-			isa += 4;
-		/* The riscv,isa DT property must start with rv64 or rv32 */
-		if (temp == isa)
+		if (IS_ENABLED(CONFIG_32BIT) && strncasecmp(isa, "rv32", 4))
 			continue;
+
+		if (IS_ENABLED(CONFIG_64BIT) && strncasecmp(isa, "rv64", 4))
+			continue;
+
+		isa += 4;
+
 		bitmap_zero(this_isa, RISCV_ISA_EXT_MAX);
 		for (; *isa; ++isa) {
 			const char *ext = isa++;