char: hpet: Remove unused variable 'm'

Fixes the following W=1 kernel build warning(s):

 drivers/char/hpet.c: In function ‘hpet_interrupt’:
 drivers/char/hpet.c:159:17: warning: variable ‘m’ set but not used [-Wunused-but-set-variable]

Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: Bob Picco <robert.picco@hp.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210520121347.3467794-13-lee.jones@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>