riscv: cpufeature: Use pre-defined ISA ext macros to index isa2hwcap

We have pre-defined ISA extension macros, here use those macros to
replace a magic number for isa2hwcap definition and some array
indexing for isa2hwcap access.

This doesn't change the original functionality, just improve the code
maintainability and readability.

Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://patch.msgid.link/20260506132152.53239-1-hui.wang@canonical.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: jason77-wang

arch/riscv/kernel/cpufeature.c

@@ -1102,16 +1102,16 @@ early_param("riscv_isa_fallback", riscv_isa_fallback_setup);
 void __init riscv_fill_hwcap(void)
 {
 	char print_str[NUM_ALPHA_EXTS + 1];
-	unsigned long isa2hwcap[26] = {0};
+	unsigned long isa2hwcap[RISCV_ISA_EXT_BASE] = {0};
 	int i, j;
 
-	isa2hwcap['i' - 'a'] = COMPAT_HWCAP_ISA_I;
-	isa2hwcap['m' - 'a'] = COMPAT_HWCAP_ISA_M;
-	isa2hwcap['a' - 'a'] = COMPAT_HWCAP_ISA_A;
-	isa2hwcap['f' - 'a'] = COMPAT_HWCAP_ISA_F;
-	isa2hwcap['d' - 'a'] = COMPAT_HWCAP_ISA_D;
-	isa2hwcap['c' - 'a'] = COMPAT_HWCAP_ISA_C;
-	isa2hwcap['v' - 'a'] = COMPAT_HWCAP_ISA_V;
+	isa2hwcap[RISCV_ISA_EXT_i] = COMPAT_HWCAP_ISA_I;
+	isa2hwcap[RISCV_ISA_EXT_m] = COMPAT_HWCAP_ISA_M;
+	isa2hwcap[RISCV_ISA_EXT_a] = COMPAT_HWCAP_ISA_A;
+	isa2hwcap[RISCV_ISA_EXT_f] = COMPAT_HWCAP_ISA_F;
+	isa2hwcap[RISCV_ISA_EXT_d] = COMPAT_HWCAP_ISA_D;
+	isa2hwcap[RISCV_ISA_EXT_c] = COMPAT_HWCAP_ISA_C;
+	isa2hwcap[RISCV_ISA_EXT_v] = COMPAT_HWCAP_ISA_V;
 
 	if (!acpi_disabled) {
 		riscv_fill_hwcap_from_isa_string(isa2hwcap);