gdb/riscv: Use correct regnum in riscv_linux_nat_target::fetch_registers

gdb/riscv: Use correct regnum in riscv_linux_nat_target::fetch_registers

In riscv_linux_nat_target::fetch_registers, if we are asked to supply
all registers (regnum parameter is -1), then we currently end up
calling regcache::raw_supply_zeroed with the regnum -1, which is
invalid.  Instead we should be passing the regnum of the specific
register we wish to supply zeroed, in this case RISCV_CSR_MISA_REGNUM.

I removed the extra { ... } block in line with the coding standard
while editing this area.

gdb/ChangeLog:

	* riscv-linux-nat.c (riscv_linux_nat_target::fetch_registers):
	Pass correct regnum to raw_supply_zeroed.