[PowerPC] Fix two if statements in gdb/ppc-linux-nat.c

[PowerPC] Fix two if statements in gdb/ppc-linux-nat.c

This patch changes two if statements to else if statements in
ppc-linux-nat.c:fetch_register for clarity.

gdb/ChangeLog:
2018-10-26  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* ppc-linux-nat.c (fetch_register): Change if statement to else
	if.
	(store_register): Likewise.