gdb/riscv: Remove redundant code, and catch more errors when accessing MISA

gdb/riscv: Remove redundant code, and catch more errors when accessing MISA

When reading the MISA register, the RISC-V specification says that, if
MISA can't be found then a default value of 0 should be assumed.

As such, this patch ensures that GDB ignores errors when accessing
both the new and old locations for the MISA register.

Additionally, this patch removes an unneeded flag parameter which
didn't provide any additional functionality beyond checking the MISA
for the default value of 0.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_read_misa_reg): Update comment, remove
	READ_P parameter, catch and ignore register access errors from
	either the old or new MISA location.
	(riscv_has_feature): Update call to riscv_read_misa_reg.