Fix unused variable warning in sfxge's ef10_tx.c

With clang 15, the following -Werror warning is produced:

    sys/dev/sfxge/common/ef10_tx.c:345:15: error: variable 'eqp' set but not used [-Werror,-Wunused-but-set-variable]
            efx_qword_t *eqp;
                         ^

The 'eqp' variable is passed as an argument to the EFSYS_BAR_WC_WRITEQ()
macro, but currently this macro ignores the argument, similar to its
other _esbp argument. Silence the warning by casting the _eqp argument
to void in the macro.

MFC after:	3 days

sys/dev/sfxge/common/efsys.h

@@ -846,6 +846,7 @@ typedef struct efsys_bar_s {
 		    ("not power of 2 aligned"));			\
 									\
 		(void) (_esbp);						\
+		(void) (_eqp);						\
 									\
 		/* FIXME: Perform a 64-bit write */			\
 		KASSERT(0, ("not implemented"));			\