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

(cherry picked from commit a79731b9453320af6fcf5ec16fa5de900dadbce8)