gdbscm_memory_port_write: use local variable to avoid adding casts

gdbscm_memory_port_write: use local variable to avoid adding casts

By having a local variable of type (const gdb_byte *), we can avoid adding
two casts.

gdb/ChangeLog:

	* guile/scm-ports.c (gdbscm_memory_port_write): Declare new
	"data" local variable and use it.