rtc: tegra: remove set but unused variable

rtc: tegra: remove set but unused variable

Fix the following warning:
drivers/rtc/rtc-tegra.c: In function ‘tegra_rtc_read_time’:
drivers/rtc/rtc-tegra.c:106:11: warning: variable ‘msec’ set but not used [-Wunused-but-set-variable]

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20191122102212.400158-8-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>