RISC-V: Fix inconsistent error message (range)

This commit fixes inconsistent error message format involving compressed
funct<n> fields.  In specific, funct6 had an error message with range
0..2^<n> ("0..64") unlike other funct<n> fields with 0..2^<n>-1
(e.g. funct4 with "0..15").

gas/ChangeLog:

	* config/tc-riscv.c (riscv_ip): Fix inconsistent error message.